Add pytest flags
Add pytest flags

Summary:
Pytest functionality can be customized through flags on command line.
Although we cannot customize command line flags easily through buck, we can also customize pytest through PYTEST_ADDOPTS env variable.
Add flags to our pytest macros in fbsource, buck, and fbcode.

Currently introduced flags are:
1. `-v`: Verbose assertion outputs on failures
2. -`p no:cacheprovider`: Prevents pytest from creating __pycache__ directories in fbsource.
3. `--continue-on-collection-errors`: Terminate immediately on collection error
4. `--color=no`: Test runners already override default pytest colors

Reviewed By: bobyangyf

fbshipit-source-id: 5b5a1e07b5060a75aeb85167749c6b5e656a0d4b

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