t3903: reduce dependencies on previous tests

t3903: reduce dependencies on previous tests

Skipping previous tests to work through only failing tests with
arguments like --run=4,122- causes some tests to fail because subdir
doesn't exist yet (it is created by a previous test; typically
"unstashing in a subdirectory"). Create it on demand for tests that need
it, but don't fail (-p) if the directory already exists.

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>