feat(upgrade): provide unit test helpers for wiring up injectors (#16848)

Adds two new helper functions that can be used when unit testing Angular services
that depend upon upgraded AngularJS services, or vice versa.
The functions return a module (AngularJS or NgModule) that is configured to wire up
the Angular and AngularJS injectors without the need to actually bootstrap a full
hybrid application.

This makes it simpler and faster to unit test services.

PR Close #16848

packages/upgrade/static/testing/BUILD.bazel

@@ -0,0 +1,19 @@
+load("//tools:defaults.bzl", "ng_module")
+
+package(default_visibility = ["//visibility:public"])
+
+exports_files(["package.json"])
+
+ng_module(
+    name = "testing",
+    srcs = glob(
+        [
+            "*.ts",
+            "src/*.ts",
+        ],
+    ),
+    deps = [
+        "//packages/core/testing",
+        "//packages/upgrade/src/common",
+    ],
+)