[appmgr] Fix RealmFakeLoaderTest.CreateWebComponent_HTTP(S)

Test isn't hermetic and was failing if web_runner package was not
present on device.

Also updates web_runner_prototype runner_url in Realm to use
fuchsia-pkg URL.

Test: appmgr_integration_tests
CF-156 #comment

Change-Id: I616355dcf67ad56aaa24f4b64da772c6dbf662dc
3 files changed