t5510-fetch: run non-httpd-specific test before sourcing 'lib-httpd.sh'

t5510-fetch: run non-httpd-specific test before sourcing 'lib-httpd.sh'

't5510-fetch.sh' sources 'lib-httpd.sh' near the end to run a
httpd-specific test, but 'lib-httpd.sh' skips all the rest of the test
script if the dependencies for running httpd tests are not fulfilled.
Alas, recently cdbd70c437 (fetch: add --[no-]show-forced-updates
argument, 2019-06-18) appended a non-httpd-specific test at the end,
and this test is then skipped as well when httpd tests can't be run.

Move this new test earlier in the test script, before 'lib-httpd.sh'
is sourced, so it will be run even when httpd tests aren't.

Also add a comment at the end of this test script to warn against
adding non-httpd-specific tests at the end, in the hope that it will
help prevent similar issues in the future.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>