unix/variants/coverage: Add test for manifest freeze_mpy().

This uses the frozentest.mpy that is also used by ports/minimal.

Also fixes two bugs that these new tests picked up:
 - File extension matching in manifestfile.py.
 - Handling of freeze_mpy results in makemanifest.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

tests/unix/extra_coverage.py

@@ -96,3 +96,8 @@ import frzmpy3
 from frzqstr import returns_NULL
 
 print(returns_NULL())
+
+# test for freeze_mpy
+import frozentest
+
+print(frozentest.__file__)