gh-134248 test_getallocatedblocks pre-check to ignore immortalized strings

[tpburns]

When sanity checking against gettotalrefcount we exclude the blocks for immortalized strings since their references are not tracked/reported. This now matches refleak.py's book-keeping using the same functions.

• This will avoid the test dependency failures seen in Python 3.15.0: test_sys.test_getallocatedblocks() fails if run after test_collections.test_odd_sizes() #134248.
• It seems to no longer have issues for free threading builds as well (and I did observe failures locally when the check was ran but didn't subtract the immortalized count). But, I don't know enough to claim it's actually solving the issue from the code comment in Make the specializing interpreter thread-safe in --disable-gil builds #115999.
• It may or may not solve the ios CI failure seen in test_sys.SysModuleTest.get_allocated_blocks test precondition intermittently fails on iOS #130384, I would appreciate guidance on how testing that would work.

As such, the current state of the PR is that the check is never skipped - to be be adjusted as needed given CI results and/or feedback.

• Issue: Python 3.15.0: test_sys.test_getallocatedblocks() fails if run after test_collections.test_odd_sizes() #134248

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.