[modular][testing] Teach TestHarness to inject services.

This CL implements features which were needed to enable using Flutter
Driver under the test harness.

Fixes:
==
* Fix test_session_shell to implement |SessionShell| and hold onto views
plumbed from AttachView; these keeps views alive and lets test harness
run flutter driver tests. This is tested by a CL using flutter driver in
 //topaz.

* Give each test harness environment a random name, so one test harness
doesn't conflict with another if both happen to be running.

New features:
==
a) TestHarness accepts a mapping of Service name -> component URL to
inject services in the hermetic environment it creates. It works
similarly to fuchsia.test CMX facet's injected services. This is
provided using TestHarnessSpec.injected_services
  - This is used to let flutter driver tests specify, and prevent
  modular_test_harness.cmx (and the test component), declaring services
  they want to use from the environment.

b) Introduce TestHarness.ConnectToEnvironmentService, which lets you
connect to environment services injected using a).

a) and b) are tested together in the TestHarnessImplTest.InjectServices
test.

Other:
* Rename TestHarness.GetService -> TestHarness.ConnectToModularService
* Updating existing tests to use injected services.

Change-Id: If940c8173f04eea36ee71934cf37e6e2a5b67834