blame tests: don't rely on t/t4018/ directory

blame tests: don't rely on t/t4018/ directory

Refactor a test added in 9466e3809d (blame: enable funcname blaming
with userdiff driver, 2020-11-01) so that the blame tests don't rely
on stealing the contents of "t/t4018/fortran-external-function".

I have another patch series that'll possibly (or not) refactor that
file, but having this test inter-dependency makes things simple in any
case by making this test more readable.

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