fix listener and route behavior when changing ParentRef

[mikemorris]

Changes proposed in this PR:

Fixes the following issue:

• Prior gateway listener doesn't update attachedRoutes back to zero when last route removes a ParentRef for that gateway

Updates test to document currently-expected behavior but not fixed yet - after discussing with @nathancoleman we believe support for multiple gateways is not yet implemented rather than a "bug":

• An HTTPRoute fails to route traffic from a second ParentRef gateway to a backend service (503 reponses with no healthy upstream) until the first ParentRef is removed
  • Traffic does still route successfully through the first gateway after adding the second gateway ParentRef

How I've tested this PR:

Written e2e test assertions that demonstrate broken behavior when changing the ParentRef of a route.

How I expect reviewers to test this PR:

Confirm that new test assertions demonstrate intended functionality, review fixes.

Checklist:

• Tests added
• CHANGELOG entry added

  Run make changelog-entry for guidance in authoring a changelog entry, and
  commit the resulting file, which should have a name matching your PR number.
  Entries should use imperative present tense (e.g. Add support for...)