unix: comment _testinternalcapi extension

As part of this commit, I verified that CPython 3.7.7 and 3.8.2
both have clean test runs on macOS! There are no new skips in 3.8
that need to be annotated.

Author: indygreg

docs/status.rst

@@ -165,4 +165,4 @@ test_winreg
 test_winsound
    No module named 'winsound'
 test_zipfile64
-   test requires loads of disk-space bytes and a long time to run
+   test requires loads of disk-space bytes and a long time to run

pythonbuild/cpython.py

@@ -37,6 +37,8 @@
     b"_socket",
     b"_statistics",
     b"_struct",
+    # Despite being a test module, this needs to be built as a
+    # built-in in order to facilitate testing.
     b"_testinternalcapi",
     b"_weakref",
     b"array",