gh-149879: Fix test_c_stack_unwind on Cygwin

[vstinner]

On Cygwin, the Python library is called "cygpython3.16.dll".

• Issue: Enhance Cygwin support #149879

[vstinner]

Sadly, this change is not enough to fix all test_c_stack_unwind tests. A test still fails on Cygwin:

======================================================================
FAIL: test_manual_unwind_finds_expected_frames (test.test_c_stack_unwind.ManualStackUnwindTests.test_manual_unwind_finds_expected_frames) (env={'PYTHON_JIT': '0'})
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/vstinner/cpython/Lib/test/test_c_stack_unwind.py", line 285, in test_manual_unwind_finds_expected_frames
self.assertGreaterEqual(
~~~~~~~~~~~~~~~~~~~~~~~^
python_frames,
^^^^^^^^^^^^^^
STACK_DEPTH,
^^^^^^^^^^^^
f"expected to find Python frames on {self.machine} with env {env}",
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
AssertionError: 4 not greater than or equal to 10 : expected to find Python frames on x86_64 with env {'PYTHON_JIT': '0'}

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"Sanitizers / TSan (free-threading)" CI failed with:

...
0:24:53 load avg: 1.04 running (1): test_free_threading (10 min 8 sec)
0:25:23 load avg: 1.02 running (1): test_free_threading (10 min 38 sec)
Error: Process completed with exit code 143.

"Ubuntu (free-threading) / build and test (ubuntu-24.04)" CI job failed with:

...
0:14:48 load avg: 8.63 [448/505] test_str passed -- running (2): test_profiling (9 min 10 sec), test_free_threading (4 min 42 sec)
0:14:49 load avg: 8.63 [449/505] test_enumerate passed -- running (2): test_profiling (9 min 11 sec), test_free_threading (4 min 43 sec)
0:14:51 load avg: 8.63 [450/505] test_copyreg passed -- running (2): test_profiling (9 min 13 sec), test_free_threading (4 min 45 sec)
make: *** [Makefile:2434: ci] Terminated
Error: Process completed with exit code 143.

I restarted the two jobs (with debug logging).

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"JIT / x86_64-pc-windows-msvc/msvc (Debug) (pull_request)" failed with: #149335

test_short_halfway_cases (test.test_strtod.StrtodTests.test_short_halfway_cases) ...
Assertion failed: uop_buffer_remaining_space(trace) > space_needed, file D:\a\cpython\cpython\Python\optimizer.c, line 902
Fatal Python error: Aborted
Current thread 0x000016b0 (most recent call first):
File "D:\a\cpython\cpython\Lib\test\test_strtod.py", line 75 in strtod
File "D:\a\cpython\cpython\Lib\test\test_strtod.py", line 101 in check_strtod
File "D:\a\cpython\cpython\Lib\test\test_strtod.py", line 142 in test_short_halfway_cases

"iOS" failed: test_tracemalloc.test_tracemalloc_track_race() crashed with a segfault.