test-lib: reorder and include GIT-BUILD-OPTIONS a lot earlier

test-lib: reorder and include GIT-BUILD-OPTIONS a lot earlier

This dot-sources GIT-BUILD-OPTIONS a lot earlier in test-lib.sh so
that its use of "perl" can use "$PERL_PATH" to choose the version of
Perl the user told us is suitable for our use.

This is iffy; I didn't check it very carefully, and I would not be
surprised if there are subtle breakages.

Signed-off-by: Junio C Hamano <gitster@pobox.com>