[sonic-platform-common] Enable pytest during build for Python 3 package (#6442)

**- Why I did it**

To enable running Pytest unit tests before building the Python 3 sonic-platform-common package

**- How I did it**

- Add Python 3 sonic-config-engine package as a dependency of Python 3 sonic-platform-common package (needed for both runtime and unit tests)
- No longer disable unit tests when building Python 3 sonic-platform-common package

Author: jleveque

rules/sonic-platform-common.mk

@@ -10,8 +10,7 @@ SONIC_PYTHON_WHEELS += $(SONIC_PLATFORM_COMMON_PY2)
 SONIC_PLATFORM_COMMON_PY3 = sonic_platform_common-1.0-py3-none-any.whl
 $(SONIC_PLATFORM_COMMON_PY3)_SRC_PATH = $(SRC_PATH)/sonic-platform-common
 $(SONIC_PLATFORM_COMMON_PY3)_PYTHON_VERSION = 3
-$(SONIC_PLATFORM_COMMON_PY3)_DEPENDS += $(SONIC_PY_COMMON_PY3)
+$(SONIC_PLATFORM_COMMON_PY3)_DEPENDS += $(SONIC_PY_COMMON_PY3) $(SONIC_CONFIG_ENGINE_PY3)
 # Synthetic dependency just to avoid race condition
 $(SONIC_PLATFORM_COMMON_PY3)_DEPENDS += $(SONIC_PLATFORM_COMMON_PY2)
-$(SONIC_PLATFORM_COMMON_PY3)_TEST = n
 SONIC_PYTHON_WHEELS += $(SONIC_PLATFORM_COMMON_PY3)