[python tests] Fix determine-reboot-cause_test.py importing module sonic_platform

This test, `determine-reboot-cause_test.py` and the script it tests,
`scripts/determine-reboot-cause` expect the module `sonic_platform` to not exist
on the system during unit tests on build hosts. The script implementation
handles this situation by trying to import `sonic_platform` in the funciton
calls, and having them return fake values in an `ImportError` exception handler.
Otherwise, if `sonic_platform` is present in the build system when running unit
tests, it will throw exceptions because the platform code is designed to run an
a real SONiC device host.

However, on the build system, there are races in the build, and it is possible
for `sonic_platform` to be installed and loaded. This will break this test.

This PR adds a fixture to remove the module and block it using temporary
monkeypatching. `None` is assigned to `system.modules['sonic_platform']` to
evict any cached loaded modules and block future loading, until the test is
complete.

This PR is in response to the following unit test failures appearing during
builds on some occasions:

```
  FAILED tests/determine-reboot-cause_test.py::TestDetermineRebootCause::test_find_hardware_reboot_cause_not_installed_or_not_implemented - TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType'
  FAILED tests/determine-reboot-cause_test.py::TestDetermineRebootCause::test_determine_reboot_cause_main_without_reboot_cause_dir - TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType'
  FAILED tests/determine-reboot-cause_test.py::TestDetermineRebootCause::test_determine_reboot_cause_main_with_reboot_cause_dir - TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType'
```

With details similar to:

```
  _ TestDetermineRebootCause.test_find_hardware_reboot_cause_not_installed_or_not_implemented _

  self = <tests.determine-reboot-cause_test.TestDetermineRebootCause object at 0x7f9c4a940d50>

      def test_find_hardware_reboot_cause_not_installed_or_not_implemented(self):
  >       result = determine_reboot_cause.find_hardware_reboot_cause()

  tests/determine-reboot-cause_test.py:140:
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
  scripts/determine-reboot-cause:127: in find_hardware_reboot_cause
      hardware_reboot_cause_major, hardware_reboot_cause_minor = get_reboot_cause_from_platform()
  scripts/determine-reboot-cause:115: in get_reboot_cause_from_platform
      platform  = sonic_platform.platform.Platform()
  /usr/local/lib/python3.13/dist-packages/sonic_platform/platform.py:34: in __init__
      if DeviceDataManager.get_dpu_count():
  /usr/local/lib/python3.13/dist-packages/sonic_platform/utils.py:183: in _impl
      method.return_value = method(*args, **kwargs)
  /usr/local/lib/python3.13/dist-packages/sonic_platform/device_data.py:372: in get_dpu_count
      dpu_data = cls.get_platform_dpus_data()
  /usr/local/lib/python3.13/dist-packages/sonic_platform/utils.py:183: in _impl
      method.return_value = method(*args, **kwargs)
  /usr/local/lib/python3.13/dist-packages/sonic_platform/device_data.py:357: in get_platform_dpus_data
      platform_path = device_info.get_path_to_platform_dir()
  /usr/local/lib/python3.13/dist-packages/sonic_py_common/device_info.py:283: in get_path_to_platform_dir
      platform_path_host = os.path.join(HOST_DEVICE_PATH, platform)
  <frozen posixpath>:90: in join
      ???
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

  funcname = 'join', args = ('/usr/share/sonic/device', None), hasstr = True
  hasbytes = False, s = None

  >   ???
  E   TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType'

  <frozen genericpath>:188: TypeError
```

Signed-off-by: Judson Wilson <[email protected]>

Author: judsonwilson-nvidia

tests/determine-reboot-cause_test.py

@@ -9,6 +9,21 @@
 from swsscommon import swsscommon
 from sonic_py_common.general import load_module_from_source
 
+
+# The platform-specific module `sonic_platform` may or may not be installed on the system hosting
+# the tests. In the build containers, it may coincidentally be installed, depending on the build
+# order. But, this test and the modules it tests are designed for the `sonic_platform` module to be
+# absent in the test environment. E.g., code that depends on DeviceInfo is bypassed when
+# `import sonic_platform` fails; otherwise, if it succeeds, and the environment is not set up like
+# a SONiC device host, the platform information may not be available and exceptions are thrown.
+# So remove the module if it's imported, and set to None to block a future import.
+@pytest.fixture(scope="module", autouse=True)
+def _remove_sonic_platform():
+    with pytest.MonkeyPatch.context() as mp:
+        mp.setitem(sys.modules, 'sonic_platform', None)
+        yield
+
+
 # TODO: Remove this if/else block once we no longer support Python 2
 if sys.version_info.major == 3:
     from unittest import mock