tests: make GIT_TEST_FAIL_PREREQS a boolean

tests: make GIT_TEST_FAIL_PREREQS a boolean

Change the GIT_TEST_FAIL_PREREQS variable from being "non-empty?" to
being a more standard boolean variable. I recently added the variable
in dfe1a17df9 ("tests: add a special setup where prerequisites fail",
2019-05-13), having to add another "non-empty?" special-case is what
prompted me to write the "git env--helper" utility being used here.

Converting this one is a bit tricky since we use it so early and
frequently in the guts of the test code itself, so let's set a
GIT_TEST_FAIL_PREREQS_INTERNAL which can be tested with the old "test
-n" for the purposes of the shell code, and change the user-exposed
and documented GIT_TEST_FAIL_PREREQS variable to a boolean.

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