Refactor IdentityManager unittests to lessen duplicate code.

This CL just extracts some commonly-used logic into helper functions
in preparation for introducing several more unittests that use this
logic extremely heavily.

There is more such extraction that could be performed for different
pieces of code; this CL just focuses on the pieces that are relevant
for the upcoming unittests.

Change-Id: I83f86faa4679dc0639cc1abff561b640baff5816
Reviewed-on: https://chromium-review.googlesource.com/1101687
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569233}

services/identity/BUILD.gn

@@ -42,6 +42,7 @@ source_set("tests") {
     "//components/sync_preferences:test_support",
     "//mojo/public/cpp/bindings:bindings",
     "//services/identity/public/cpp",
+    "//services/identity/public/cpp:test_support",
     "//services/identity/public/mojom",
     "//services/service_manager/public/cpp",
     "//services/service_manager/public/cpp:service_test_support",