tint/tests: Actually validate the generated WGSL

We weren't actually enabling WGSL validating in Tint in the E2E test
runner.

Mark a few tests as SKIP for cases that do not actually validate.

Change-Id: I62a55ce701f704d744c32f2cd5cb97683e270e96
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/117960
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: James Price <jrprice@google.com>
Kokoro: Ben Clayton <bclayton@google.com>

tools/src/cmd/test-runner/main.go

@@ -607,6 +607,7 @@ func (j job) run(cfg runConfig) {
 		validate := false
 		switch j.format {
 		case wgsl:
+			args = append(args, "--validate") // wgsl validation uses Tint, so is always available
 			validate = true
 		case spvasm, glsl:
 			args = append(args, "--validate") // spirv-val and glslang are statically linked, always available