py3: add a test that proves %include and missing-extension code works

tests/test-check-py3-commands.t

@@ -1,14 +1,25 b''
 #require py3exe
 
 This test helps in keeping a track on which commands we can run on
 Python 3 and see what kind of errors are coming up.
 The full traceback is hidden to have a stable output.
 
   $ for cmd in version debuginstall ; do
   >   echo $cmd
   >   $PYTHON3 `which hg` $cmd 2>&1 2>&1 | tail -1
   > done
   version
   warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
   debuginstall
   TypeError: Can't convert 'bytes' object to str implicitly
+
+  $ cat > included-hgrc <<EOF
+  > [extensions]
+  > babar = imaginary_elephant
+  > EOF
+  $ cat >> $HGRCPATH <<EOF
+  > %include $TESTTMP/included-hgrc
+  > EOF
+  $ $PYTHON3 `which hg` version | tail -1
+  *** failed to import extension babar from imaginary_elephant: *: 'imaginary_elephant' (glob)
+  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.