Re-enable skipped test_zlib tests on s390x hardware acceleration

[encukou]

Some variants of the s390x platform have instructions for hardware-accelerated deflate compression. With HW acceleration, compressed byte stream can be different from the software implementation in zlib. (It still decompresses to the original of course.)
The zlib-ng library can be built to use this, and CPython can be built with zlib-ng.

In 2022 (#90781, GH-31096), two tests that failed on s390x were unconditionally skipped. IMO, it would be better if we only skip checking the compressed stream, but do check the round-trip result.
Testing should be a bit easier now that a buildbot worker has the HW-accelerated zlib.

In #107535, the skip_on_s390x variable was separated from the comment that explains it.
I propose to name the skip condition HW_ACCELERATED rather than skip_on_s390x -- theoretically, other platforms might need this in the future.

Linked PRs

• gh-125041: test_zlib: For s390x HW acceleration, only skip checking the compressed bytes #125042
• [3.12] gh-125041: test_zlib: For s390x HW acceleration, only skip checking the compressed bytes (GH-125042) #125526
• [3.13] gh-125041: test_zlib: For s390x HW acceleration, only skip checking the compressed bytes (GH-125042) #125527
• [3.11] gh-125041: test_zlib: For s390x HW acceleration, only skip checking the compressed bytes (GH-125042) #125577
• [3.10] gh-125041: gh-90781: test_zlib: For s390x HW acceleration, skip checking the compressed bytes (GH-125042) #125585
• [3.9] gh-125041: gh-90781: test_zlib: For s390x HW acceleration, skip checking the compressed bytes (GH-125042) #125587