userdiff: remove support for "broken" tests

There have been no "broken" tests since 75c3b6b2e8 (userdiff: improve
Fortran xfuncname regex, 2020-08-12). Let's remove the test support
for them.

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

t/t4018-diff-funcname.sh

@@ -94,13 +94,7 @@ test_expect_success 'setup hunk header tests' '
 # check each individual file
 for i in $(git ls-files)
 do
-	if grep broken "$i" >/dev/null 2>&1
-	then
-		result=failure
-	else
-		result=success
-	fi
-	test_expect_$result "hunk header: $i" "
+	test_expect_success "hunk header: $i" "
 		git diff -U1 $i >actual &&
 		grep '@@ .* @@.*RIGHT' actual
 	"

t/t4018/README

@@ -7,9 +7,6 @@ at least two lines from the line that must appear in the hunk header.
 The text that must appear in the hunk header must contain the word
 "right", but in all upper-case, like in the title above.
 
-To mark a test case that highlights a malfunction, insert the word
-BROKEN in all lower-case somewhere in the file.
-
 This text is a bit twisted and out of order, but it is itself a
 test case for the default hunk header pattern. Know what you are doing
 if you change it.