test-lib: introduce required prereq for test runs

In certain environments or for specific test scenarios we might expect a
specific prerequisite check to succeed. Therefore we would like to abort
running our tests if this is not the case.

To remedy this we add the environment variable GIT_TEST_REQUIRE_PREREQ
which can be set to a space separated list of prereqs. If one of these
prereq tests fail then the whole test run will abort.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -466,6 +466,12 @@ explicitly providing repositories when accessing submodule objects is
 complete or needs to be abandoned for whatever reason (in which case the
 migrated codepaths still retain their performance benefits).
 
+GIT_TEST_REQUIRE_PREREQ=<list> allows specifying a space speparated list of
+prereqs that are required to succeed. If a prereq in this list is triggered by
+a test and then fails then the whole test run will abort. This can help to make
+sure the expected tests are executed and not silently skipped when their
+dependency breaks or is simply not present in a new environment.
+
 Naming Tests
 ------------
 

t/test-lib-functions.sh

@@ -680,6 +680,17 @@ test_have_prereq () {
 			# Keep a list of missing prerequisites; restore
 			# the negative marker if necessary.
 			prerequisite=${negative_prereq:+!}$prerequisite
+
+			# Abort if this prereq was marked as required
+			if test -n "$GIT_TEST_REQUIRE_PREREQ"
+			then
+				case " $GIT_TEST_REQUIRE_PREREQ " in
+				*" $prerequisite "*)
+					BAIL_OUT "required prereq $prerequisite failed"
+					;;
+				esac
+			fi
+
 			if test -z "$missing_prereq"
 			then
 				missing_prereq=$prerequisite