t7406: prefer test_* helper functions to test -[feds]

t7406: prefer test_* helper functions to test -[feds]

test -e, test -s, etc. do not provide nice error messages when we hit
test failures, so use the test_* helper functions from
test-lib-functions.sh.

Also, add test_path_exists() to test-lib-function.sh while at it, so
that we don't need to worry whether submodule/.git is a file or a
directory.  It currently is a file with contents of the form
   gitdir: ../.git/modules/submodule
but it could be changed in the future to be a directory; this test
only really cares that it exists.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>