Feat(CI): Use environment variable to specify the location of testing data

[acezen]

Reason for this PR

as issue #510 describes, since the released source code tarball would not contains the submodules, to make the test work with tarball, it's better to deprecate testing data submodule and use developer setting environment variable to indicate the location of testing data.

What changes are included in this PR?

• refine the unit test of c++/scala library with the 'GAR_TEST_DATA' env variable
• update the related developer document
• [TODO] Drop the testing submodule until java/python library update the unit test like cpp/scala. (java/python not bump up in this release)

Are these changes tested?

already tested by current test case.

Are there any user-facing changes?

no

[acezen]

Hi， @lixueclaire @SemyonSinchenko ， This PR may need your review, thanks.