BUILD.gn: Fix gn check error

The 'tint_unittests_src' source_set needed to depend on gmock_and_gtest,
which is normally done with the 'tint_unittests_source_set' template,
but this is the root source_set for the unit tests.
To fix, these root-level sources have been moved to
'tint_unittests_core_src' which uses the 'tint_unittests_source_set'
template.

Change-Id: I0d76c55c744646fd7deff22406a668f8c6cdf519
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/69220
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>
Kokoro: Ben Clayton <bclayton@google.com>

test/BUILD.gn

@@ -686,9 +686,7 @@ tint_unittests_source_set("tint_unittests_glsl_writer_src") {
   ]
 }
 
-source_set("tint_unittests_src") {
-  testonly = true
-
+tint_unittests_source_set("tint_unittests_core_src") {
   sources = [
     "../src/block_allocator_test.cc",
     "../src/castable_test.cc",
@@ -706,6 +704,15 @@ source_set("tint_unittests_src") {
 
   deps = [
     ":tint_unittests_ast_src",
+  ]
+}
+
+source_set("tint_unittests_src") {
+  testonly = true
+
+  deps = [
+    ":tint_unittests_ast_src",
+    ":tint_unittests_core_src",
     ":tint_unittests_diagnostic_src",
     ":tint_unittests_inspector_src",
     ":tint_unittests_resolver_src",