Get `gn gen --check` actually working in repo

Fixes an issue in the .gn that allows the check to run as expected. As
well as fixes issues discovered from running the check.

BUG=tint:735

Change-Id: I48cd01339d06132933c3da4f0e9d45cdbaff3b55
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/48700
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

.gn

@@ -14,4 +14,4 @@
 
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
-check_targets = [ "//:*" ]
+check_targets = [ "//*" ]

test/BUILD.gn

@@ -88,7 +88,12 @@ source_set("tint_unittests_main") {
     sources = [ "//gpu/tint_unittests_main.cc" ]
   } else {
     sources = [ "../src/test_main.cc" ]
-    deps += [ "${tint_root_dir}/src:libtint" ]
+    deps += [
+      ":tint_test_helpers",
+      ":tint_unittests_hlsl_writer_src",
+      ":tint_unittests_msl_writer_src",
+      "${tint_root_dir}/src:libtint",
+    ]
   }
 }