test_capi.test__testcapi() becomes more verbose (#4197)

Write the name of each subtest on a new line to help debugging when
a test does crash Python.

Lib/test/test_capi.py

@@ -751,9 +751,14 @@ class TestThreadState(unittest.TestCase):
 
 class Test_testcapi(unittest.TestCase):
     def test__testcapi(self):
+        if support.verbose:
+            print()
         for name in dir(_testcapi):
-            if name.startswith('test_'):
+            if not name.startswith('test_'):
+                continue
             with self.subTest("internal", name=name):
+                if support.verbose:
+                    print(f"  {name}", flush=True)
                 test = getattr(_testcapi, name)
                 test()