Run Python OP tests in a single Python process to improve test time.

[panyx0718]

Currently, our tests run with 2 GPUs, the init time is absurdly long:
about 4s for each process. Currently, we run each OP test on
different processes. This PR:

1. create cmake function py_test_modules which will generate the
   Makefile that runs a list of Python unittest module in a single Python
   process.

2. move all "python unittest compatible" (e.g., used the unittest
   package, not just a regular python file). from fluid/tests to
   fluid/tests/unittests.

3. cmake now will run all OP tests in fluid/tests/unittests in a
   single process, except the time-consuming tests, they are separated
   into different processes to utilize parallelism. Please make sure to
   use the unittest package if you put the python test file in
   fluid/tests/unittests

4. remove all exit(0) from fluid/tests/unittests/*.py, exit(0) is used
   to disable unittest, we can not do it when running all tests in a
   single process since it will terminate the process without running the
   other tests. Instead, the test is disabled in
   fluid/tests/unittests/CMakeLists.txt. FIXME is added for each disabled
   item. Please disable the unittest from
   fluid/tests/unittests/CMakeLists.txt, instead of adding exit(0) to the
   Python file, for all Python file in fluid/tests/unittests/.

5. add an option WITH_FAST_BUNDLE_TEST. When OFF, will run the unit
   tests in separate process so that they can be tested individually.

[CLAassistant]

All committers have signed the CLA.

[wangkuiyi]

Thanks to @panyx0718 for working over the weekend on this very urgent project!

[wangkuiyi]

I like py_test_modules and it has the potential in many other projects. For the near future, is it the only purpose of py_test_modules to bundle operators unit tests? If so, how about we move it to fluid/tests/unittests/CMakeLists.txt?

[panyx0718]

Done

[wangkuiyi]

It seems that we'd always want to run tests the fast way. How about we remove the CMake option WITH_FAST_BUNDLE_TEST by assuming it is true?

[panyx0718]

When one test within the bundled test fails, user might want to only test that failed test instead of testing all tests in the bundle (takes a long time)?

[wangkuiyi]

Good point! With the current solution, it seems that the user need to re-run cmake, like the following:

ctest # assume that a Python test failed
cmake -DWITH_FAST_BUNDLE_TEST=OFF .. # re-run cmake to swtich to the "slow" mode
make # re-build as we re-run cmake
ctest -V -R the_failed_python_test

Am I correctly understanding the situation? If so, I am afraid that the users might want a quicker solution? Is it a good idea if (1) we assume WITH_FAST_BUNDLE_TEST is always true and remove it, and (2) we add a Gflag in our Python test framework that works like the -R option of ctest?

Anyway, if this PR passes, I'd like to merge it, before we find a solution for running a specified Python test in the bundle.

[panyx0718]

The PR passes now. I can think about your proposal as a followup

[panyx0718]

Thanks, PTAL

[panyx0718]

It seems "continuous-integration/travis-ci" is not working?

[luotao1]

The travis-ci seems normally, maybe you can push a commit again.

[panyx0718]

Thanks, I resolved that conflicts and it seems to work now.