Fix selection of standalone tests

[awaelchli]

What does this PR do?

Current master branch does not run the standalone (previously known as special) tests. There is a bug with reading the STANDALONE_PATTERN environment variable. Instead of a env variable, we will now rely on standard pytest argument passing, e.g.,

bash tests/standalone_tests.sh -k deepspeed

to run all tests that have the word "deepspeed" in their name.

This fix should also be included in 1.5.5.

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Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @tchaton @carmocca @akihironitta @Borda

[tchaton]

Great catch !