Fix distutils tests on Windows (#12678).

- First, support.fixup_build_ext (already used to set proper
  library_dirs value under Unix shared builds) gains the ability to
  correctly set the debug attribute under Windows debug builds.

- Second, the filename for the extension module gets a _d suffix under
  debug builds.

- Third, the test code properly puts our customized build_ext object
  into an internal dictionary to make sure that the install command will
  later use our object instead of re-creating one.  That’s the downside
  of using low-level APIs in our test code: we have to manually push
  knobs and turn handles that would otherwise be handled behind the
  scenes.

Thanks to Nadeem for the testing.

Lib/distutils/tests/test_build_ext.py

@@ -47,10 +47,6 @@ class BuildExtTestCase(TempdirManager,
         dist.package_dir = self.tmp_dir
         cmd = build_ext(dist)
         fixup_build_ext(cmd)
-        if os.name == "nt":
-            # On Windows, we must build a debug version iff running
-            # a debug build of Python
-            cmd.debug = sys.executable.endswith("_d.exe")
         cmd.build_lib = self.tmp_dir
         cmd.build_temp = self.tmp_dir
 
@@ -305,9 +301,6 @@ class BuildExtTestCase(TempdirManager,
         cmd.ensure_finalized()
         self.assertEqual(len(cmd.get_outputs()), 1)
 
-        if os.name == "nt":
-            cmd.debug = sys.executable.endswith("_d.exe")
-
         cmd.build_lib = os.path.join(self.tmp_dir, 'build')
         cmd.build_temp = os.path.join(self.tmp_dir, 'tempt')