checker: check duplicates on match with no else

Refactor match duplication test to work even if there is not else, and to include every expression.
Add tests for duplicate expressions in match.

vlib/os/os.v

@@ -467,28 +467,30 @@ pub fn sigint_to_signal_name(si int) string {
 	$if linux {
 		// From `man 7 signal` on linux:
 		match si {
-			30, 10, 16 {
+			// TODO dependent on platform
+			// works only on x86/ARM/most others
+			10 /*, 30, 16 */ {
 				return 'SIGUSR1'
 			}
-			31, 12, 17 {
+			12 /*, 31, 17 */ {
 				return 'SIGUSR2'
 			}
-			20, 17, 18 {
+			17 /*, 20, 18 */ {
 				return 'SIGCHLD'
 			}
-			19, 18, 25 {
+			18 /*, 19, 25 */ {
 				return 'SIGCONT'
 			}
-			17, 19, 23 {
+			19 /*, 17, 23 */ {
 				return 'SIGSTOP'
 			}
-			18, 20, 24 {
+			20 /*, 18, 24 */ {
 				return 'SIGTSTP'
 			}
-			21, 21, 26 {
+			21 /*, 26 */ {
 				return 'SIGTTIN'
 			}
-			22, 22, 27 {
+			22 /*, 27 */ {
 				return 'SIGTTOU'
 			}
 			// /////////////////////////////