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>

t/test-lib-functions.sh

@@ -309,7 +309,7 @@ test_unset_prereq () {
 }
 
 test_set_prereq () {
-	if test -n "$GIT_TEST_FAIL_PREREQS"
+	if test -n "$GIT_TEST_FAIL_PREREQS_INTERNAL"
 	then
 		case "$1" in
 		# The "!" case is handled below with
@@ -1043,7 +1043,7 @@ perl () {
 # The error/skip message should be given by $2.
 #
 test_skip_or_die () {
-	if ! git env--helper --mode-bool --variable=$1 --default=0 --exit-code --quiet
+	if ! git env--helper --type=bool --default=false --exit-code $1
 	then
 		skip_all=$2
 		test_done