fix integration tests for OIDC in Charmed Kubeflow 1.7+ (#174)

refactors the integration test CI to:
* use the latest/edge version of oidc-gatekeeper
* provide oidc-gatekeeper with the public-url config during setup rather than during web browsing.  This is required because the oidc-gatekeeper v1.7+ will block the charm without this setting, and also because it makes more sense then changing oidc-gatekeeper settings in the fixture for a later test

Context: previously, the integration tests here incorrectly pinned OIDC to that from charmed kubeflow 1.4, which is not how we want our main branch testing to work.