Modules/_testcapimodule.c is a nearly-8000-line behemoth with no clear structure or organization. It is getting hard to maintain.

It also doesn't work well with testing (a) feature macros that affect Python.h and (b) module initialization, so we have additional C-API testing modules: _testmultiphase, _testimportmultiple, _testinternalcapi.
_testbuffer is already split out, but there are many other aspects of the API that would use a similar dedicated test suite.

We should split and combine these, ideally without polluting the namespace of top-level modules.

• PR: gh-93649: Split vectorcall testing from _testcapimodule.c #94549

• PR: gh-93649: Fix dependencies of _testcapi #94695

• PR: gh-93649: Fix linkage of _PyTestCapi_Init_Vectorcall #94702

• PR: gh-93649: Split heaptype tests from _testcapimodule.c  #95386

• PR: gh-93649: Undefine NDEBUG in Modules/_testcapi/* to enable assert() #95793

• PR: gh-93649: Split unicode tests from _testcapimodule.c & add some more #95819

• PR: gh-93649: Split getargs testing from _testcapimodule.c #99346

• PR: gh-93649: Split pytime and datetime tests from _testcapimodule.c #99494

• PR: gh-93649: Split memory and docstring tests from _testcapimodule.c #99517

• PR: gh-93649: Split watcher API tests from _testcapimodule.c #99532

• PR: gh-93649: Split float/long tests from _testcapimodule.c #99549

• PR: gh-93649: Split tracemalloc tests from _testcapimodule.c #99551

• PR: gh-93649: Split exception tests from _testcapimodule.c #102173

• PR: gh-93649: Split gc- and allocation tests from _testcapimodule.c #104403

• PR: gh-93649: Split _testcapimodule.c into smaller files #129400

• PR: gh-93649: Move PyFrame C API tests to test_capi #129512

• PR: gh-93649: Add Modules/_testcapi/type.c file #129516

• PR: gh-93649: Add Modules/_testcapi/function.c file #129521

• PR: gh-93649: Move _testcapi tests to specific files #129544