[OpenMP][NFC] lit: Allow setting default environment variables for test

Add CHECK_OPENMP_ENV environment variable which will be passed to environment
variables for test (make check-* target). This provides a handy way to
exercise various openmp code with different settings during development.

For example, to change default barrier pattern:
```
$ env CHECK_OPENMP_ENV="KMP_FORKJOIN_BARRIER_PATTERN=hier,hier \
KMP_PLAIN_BARRIER_PATTERN=hier,hier \
KMP_REDUCTION_BARRIER_PATTERN=hier,hier" \
ninja check-openmp
```

Even with this, each test can set appropriate environment variables if needed
as before.

Also, this commit adds missing documention about how to run tests in README.

Patch provided by t-msn

Differential Revision: https://reviews.llvm.org/D122645