test: Add a new test_expect_equal_failure

Which allows us to have a known-broken test that would otherwise use
test_expect_equal.

test/README

@@ -147,6 +147,13 @@ library for your script to use.
    will generate a failure and print the difference of the two
    strings.
 
+ test_expect_equal_failure <output> <expected>
+
+   This works similar to test_expect_equal (see above) but is used to
+   mark a test that demonstrates a known breakage, (that is, the
+   author of the test expectes "output" and "expected" to differ until
+   the breakage is fixed). See test_expect_failure for details.
+
  test_debug <script>
 
    This takes a single argument, <script>, and evaluates it only

test/test-lib.sh

@@ -414,6 +414,24 @@ test_expect_equal ()
     fi
 }
 
+test_expect_equal_failure ()
+{
+	test "$#" = 3 && { prereq=$1; shift; } || prereq=
+	test "$#" = 2 ||
+	error "bug in the test script: not 2 or 3 parameters to test_expect_equal"
+
+	output="$1"
+	expected="$2"
+	if ! test_skip "$@"
+	then
+		if [ "$output" = "$expected" ]; then
+			test_known_broken_ok_ "$test_subtest_name"
+		else
+			test_known_broken_failure_ "$test_subtest_name"
+		fi
+    fi
+}
+
 NOTMUCH_NEW ()
 {
     notmuch new | grep -v -E -e '^Processed [0-9]*( total)? file|Found [0-9]* total file'