Move tint unittest behind a build flag.

This CL moves the Tint unittest code behind a build flag in order
to hide the //testing/test.gni dependency from downstream consumers.

The unittests are enabled by default and can be disabled by setting
`tint_build_unittests` to false.

Bug: dawn:1517
Change-Id: Id560ed98416ba0725b62a93ddd27caf341699e75
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/94481
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

build_overrides/tint.gni

@@ -19,3 +19,5 @@ tint_build_spv_reader = true
 tint_build_spv_writer = true
 tint_build_wgsl_reader = true
 tint_build_wgsl_writer = true
+
+tint_build_unittests = true

tint_overrides_with_defaults.gni

@@ -71,4 +71,9 @@ declare_args() {
   if (!defined(tint_build_glsl_writer)) {
     tint_build_glsl_writer = true
   }
+
+  # Build unittests
+  if (!defined(tint_build_unittests)) {
+    tint_build_unittests = true
+  }
 }