build: create google3 migration tests bazel target (#31817)

Creates a separate bazel target for the google3 migration
tests. The benefit is that it's faster to run tests for
public migrations in development. Google3 lint rules are
usually another story/implementation and the tests are quite
slow due to how TSLint applies replacements.

Additionally if something changes in the google3 tslint rules,
the tests which aren't affected re-run unnecessarily.

PR Close #31817

packages/core/schematics/migrations/google3/BUILD.bazel

@@ -4,7 +4,7 @@ ts_library(
     name = "google3",
     srcs = glob(["**/*.ts"]),
     tsconfig = "//packages/core/schematics:tsconfig.json",
-    visibility = ["//packages/core/schematics/test:__pkg__"],
+    visibility = ["//packages/core/schematics/test/google3:__pkg__"],
     deps = [
         "//packages/core/schematics/migrations/injectable-pipe",
         "//packages/core/schematics/migrations/missing-injectable",