tests: Add test for latest patch

This tesst infloops without it.

testsuite/css/parser/Makefile.am

@@ -333,6 +333,7 @@ test_data = \
 	single-slash.css \
 	single-slash.errors\
 	single-slash.ref.css \
+	string-values.css \
 	test.png \
 	transition.css \
 	transition.ref.css \

testsuite/css/parser/string-values.css

@@ -0,0 +1,27 @@
+n {
+  font-family: "\A ";
+}
+
+r {
+  font-family: "\C ";
+}
+
+f {
+  font-family: "\D ";
+}
+
+a {
+  font-family: "\"";
+}
+
+b {
+  font-family: "\'";
+}
+
+c {
+  font-family: "'";
+}
+
+d {
+  font-family: "\\";
+}