Fixed unit-test env var list parsing and improved filtered test run speed

[corey-derochie-amd]

Details

What were the changes?

• Fixed a bug in unit-test env var list parsing.
• Made tests run faster with UT_DATATYPES and UT_REDOPS filters when a test has no remaining items.

Why were the changes made?
Improve speed and usability of unit-tests

How was the outcome achieved?

• Saved env var strings to a copy to ensure non-destructive parsing.
• Used GTEST_SKIP to bail early when UT_DATATYPES or UT_REDOPS filters make a zero-size test.

Approval Checklist

Do not approve until these items are satisfied.

• [ ] Verify the CHANGELOG has been updated, if
  • there are any NCCL API version changes,
  • any changes impact library users, and/or
  • any changes impact any other ROCm library.