Increase the test timeout of some string tests

Recent changes in test-strncasecmp and test-strncmp pushed the run time
of the tests above the 4 minute limit specified in test-string.h on an
arm tester machine.

string/test-strncasecmp.c

@@ -24,6 +24,7 @@
 
 #define TEST_MAIN
 #define TEST_NAME "strncasecmp"
+#define TIMEOUT (5 * 60)
 #include "test-string.h"
 
 typedef int (*proto_t) (const char *, const char *, size_t);