refactor(#154): use flag-name lookup in InvalidRepo and InvalidPRNumber tests

Make the override code robust against base-arg order changes: search for
the flag name ('--repo', '--pr') and replace the following value, rather
than searching for the value string itself.

This prevents a subtle bug if the same value appeared elsewhere in the
base args before the target flag.

cmd/review-bot/main_test.go

@@ -902,10 +902,10 @@ func TestMainSubprocess_InvalidRepo(t *testing.T) {
 	if os.Getenv("TEST_SUBPROCESS_MAIN") == "1" {
 		flag.CommandLine = flag.NewFlagSet(os.Args[0], flag.ExitOnError)
 		args := baseSubprocessArgs()
-		// Replace the canonical --repo with an invalid one.
+		// Replace the canonical --repo value with an invalid one.
 		for i, a := range args {
-			if a == "owner/repo" {
-				args[i] = "invalidrepo"
+			if a == "--repo" && i+1 < len(args) {
+				args[i+1] = "invalidrepo"
 				break
 			}
 		}
@@ -931,8 +931,8 @@ func TestMainSubprocess_InvalidPRNumber(t *testing.T) {
 		args := baseSubprocessArgs()
 		// Replace the canonical --pr value with a non-numeric string.
 		for i, a := range args {
-			if a == "1" {
-				args[i] = "notanumber"
+			if a == "--pr" && i+1 < len(args) {
+				args[i+1] = "notanumber"
 				break
 			}
 		}