remote-mediawiki tests: use a more idiomatic dispatch table

remote-mediawiki tests: use a more idiomatic dispatch table

Change the dispatch table code in test-gitmw.pl to use a hash where
subroutine references are the values. This is more obvious than a hash
where the values are strings we'll use to go searching around in the
symbol table for the function.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>