Support labels in pxr_register_test

[nvmkuruc]

Description of Change(s)

ctest offers labels as a way to group tests for downstream runners. This introduces support for test labeling and adds a single label flaky, implying that the test occasionally fails but usually succeeds on retry.

The initial set of flaky tests were identified by looking through recent pipeline logs for failures that appeared spurious. The work tests have been known to fail occasionally for most of the repos lifetime.

Future PRs may contribute additional labels (such as those used in regex-based exclusion rules in the CI pipelines).

Example usage:

# Exclude flaky tests
ctest -C Release -LE flaky
# Allow multiple runs of flaky tests
ctest -C Release -L flaky --repeat until-pass 3

Link to proposal (if applicable)

aousd/build-ig-initiatives#18

Fixes Issue(s)

Checklist

• I have created this PR based on the dev branch

• I have followed the coding conventions

• I have added unit tests that exercise this functionality (Reference:
  testing guidelines)

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement (Reference:
  Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-12136

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