test: move subtest variables reset into a dedicated function

Currently, there is only one such variable test_subtest_known_broken_.
But more will be added in the future.

test/test-lib.sh

@@ -429,7 +429,7 @@ test_begin_subtest ()
 	error "bug in test script: Missing test_expect_equal in ${BASH_SOURCE[1]}:${BASH_LINENO[0]}"
     fi
     test_subtest_name="$1"
-    test_subtest_known_broken_=
+    test_reset_state_
     # Remember stdout and stderr file descriptors and redirect test
     # output to the previously prepared file descriptors 3 and 4 (see
     # below)
@@ -581,14 +581,14 @@ test_failure_message_ () {
 }
 
 test_known_broken_ok_ () {
-	test_subtest_known_broken_=
+	test_reset_state_
 	test_fixed=$(($test_fixed+1))
 	say_color pass "%-6s" "FIXED"
 	echo " $@"
 }
 
 test_known_broken_failure_ () {
-	test_subtest_known_broken_=
+	test_reset_state_
 	test_broken=$(($test_broken+1))
 	test_failure_message_ "BROKEN" "$@"
 	return 1
@@ -624,7 +624,7 @@ test_skip () {
 	fi
 	case "$to_skip" in
 	t)
-		test_subtest_known_broken_=
+		test_reset_state_
 		say_color skip >&3 "skipping test: $@"
 		say_color skip "%-6s" "SKIP"
 		echo " $1"
@@ -864,6 +864,10 @@ test_emacs () {
 	emacsclient --socket-name="$EMACS_SERVER" --eval "(progn $@)"
 }
 
+test_reset_state_ () {
+	test_subtest_known_broken_=
+}
+
 
 find_notmuch_path ()
 {