gh-113787: Fix refleaks in test_capi

[neonene]

Since b2566d8, the _testcapi module has specified a non-negative value to PyModuleDef.m_size, which seems to require a bit different refcount management. Every (sub-) interpreter now calls the single phase PyInit__testcapi() with each module state. Previously, the call was only once when running the subinterpreter test case, and repeating Py_INCREF(TestError) n-times in PyInit__testcapi did not affect passing the refleak test for me.

The patch for vectorcall_limited.c follows the way in structmember.c

cc @nascheme @ericsnowcurrently @Eclips4

• Issue: test_capi has refleaks #113787

[Eclips4]

Great job! Thanks.
Refleak tests with this patch:

./python -m test -R 3:3 test_capi
Using random seed: 898642448
0:00:00 load avg: 0.02 Run 1 test sequentially
0:00:00 load avg: 0.02 [1/1] test_capi
beginning 6 repetitions
123456
......
test_capi passed in 33.7 sec

== Tests result: SUCCESS ==

1 test OK.

Total duration: 33.7 sec
Total tests: run=823 skipped=4
Total test files: run=1/1
Result: SUCCESS