test: add `test-repo` requirement to `test-check-format` (issue6395)

Kindly reported by Tristan Seligmann <[e-mail]>

tests/test-check-format.t

@@ -1,5 +1,5 b''
-#require black
+#require black test-repo
 
   $ cd $RUNTESTDIR/..
   $ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`