Fix CTest failures (#3977)

When building Git via Visual Studio and then running the tests via CTest
(which is made very easy by Visual Studio), there are test failures.
This PR intends to address those.

This closes #3966

t/test-lib.sh

@@ -51,7 +51,16 @@ then
 	TEST_OUTPUT_DIRECTORY=$TEST_DIRECTORY
 fi
 GIT_BUILD_DIR="${TEST_DIRECTORY%/t}"
-if test "$TEST_DIRECTORY" = "$GIT_BUILD_DIR"
+if test -f "$GIT_BUILD_DIR/GIT-BUILD-DIR"
+then
+	GIT_BUILD_DIR="$(cat "$GIT_BUILD_DIR/GIT-BUILD-DIR")" || exit 1
+	# On Windows, we must convert Windows paths lest they contain a colon
+	case "$(uname -s)" in
+	*MINGW*)
+		GIT_BUILD_DIR="$(cygpath -au "$GIT_BUILD_DIR")"
+		;;
+	esac
+elif test "$TEST_DIRECTORY" = "$GIT_BUILD_DIR"
 then
 	echo "PANIC: Running in a $TEST_DIRECTORY that doesn't end in '/t'?" >&2
 	exit 1