Fix tint e2e test runner not validating non-IR HLSL and MSL

Golang requires explicit "fallthrough" statements.

Change-Id: Ib128aa7026f49ccf0c424c20199c9970fdeb7b3f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/208014
Auto-Submit: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: James Price <jrprice@google.com>
diff --git a/tools/src/cmd/tests/main.go b/tools/src/cmd/tests/main.go
index 75e40ff..7de9b9e 100644
--- a/tools/src/cmd/tests/main.go
+++ b/tools/src/cmd/tests/main.go
@@ -737,20 +737,17 @@
 		case spvasm, glsl, glslIR:
 			args = append(args, "--validate") // spirv-val and glslang are statically linked, always available
 			validate = true
-		case hlslDXC:
-		case hlslDXCIR:
+		case hlslDXC, hlslDXCIR:
 			if cfg.dxcPath != "" {
 				args = append(args, "--dxc", cfg.dxcPath)
 				validate = true
 			}
-		case hlslFXC:
-		case hlslFXCIR:
+		case hlslFXC, hlslFXCIR:
 			if cfg.fxcPath != "" {
 				args = append(args, "--fxc", cfg.fxcPath)
 				validate = true
 			}
-		case msl:
-		case mslIR:
+		case msl, mslIR:
 			if cfg.xcrunPath != "" {
 				args = append(args, "--xcrun", cfg.xcrunPath)
 				validate = true