With the attached patch (the patch does reintroduce the bug in 'test_incref_decref_API' for testing purposes), it is possible to find the failing subtest rapidly:

Get the number of subtests (35 subsets):
$ export SUBTEST_RANGE="[]"
$ ./python -m test -m test__testcapi test_capi

Then run:
$ ./python -m test -m test__testcapi -R 23:23 test_capi

after modifying, each time, the range of subtests to execute, with:
$ export SUBTEST_RANGE="range(1,18)"    # tests 1-17  result: fail
$ export SUBTEST_RANGE="range(1,9)"     # test 1-8    result: pass
$ export SUBTEST_RANGE="range(1,13)"    # test 1-12   result: fail
$ export SUBTEST_RANGE="range(1,11)"    # test 1-10   result: fail
$ export SUBTEST_RANGE="[9]"            # so it must be test #9, check it now

The strong limitation with this solution is that the subTest context manager must now be enclosed in a 'try except unittest.SkipTest' clause and that the context manager is used more than 100 times in the test suite.