Remove test/tint/BUILD.gn.

This Cl moves the contents of the test/tint/BUILD.gn file into the
src/tint/BUILD.gn file. This moves the tests into the same build file as
what is being tested. This is more in line with how the CMakeLists.txt
file is setup.

This removes the need to update 2 BUILD.gn files when adding new files
and tests to tint.

Bug: tint:1517
Change-Id: I9beb8a8bff7c10ac875647246ad4362f7e60f7da
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/94040
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

BUILD.gn

@@ -22,8 +22,8 @@ group("all") {
     "src/dawn/tests",
     "src/fuzzers/dawn:dawn_fuzzers",
     "src/tint:libtint",
+    "src/tint:tint_unittests",
     "src/tint/fuzzers",
-    "test/tint:tint_unittests",
   ]
   if (dawn_standalone) {
     deps += [