gqltest: Ensure fusion test is actually cloning depot

[ryanslade]

The previous tests we not testing cloning viap4-fusion, the following was happening:

git p4 test runs, completes and deletes external service which then soft
deletes repo. p4-fusion can't find repo to delete since it's name has changed
so does nothing. Repo is still on disk so we end up fetching rather than
cloning. Even if that fails, we still find the repo on disk and mark the test
as passed.

To fix this we now:

• Delete the repo on disk before deleting the service. (Confirmed locally that the repo is removed on disk)
• Clone a different repo in each test

The tests were also refactored to make things a little more explicit.

Test plan

Tested many times locally, confirm that passed integration tests in CI.

Also manually changes code locally so that the p4-fusion command would fail
to confirm we were actually running it.

Part of https://github.com/sourcegraph/sourcegraph/pull/44301

[ryanslade]

Integration tests running here:
https://buildkite.com/sourcegraph/sourcegraph/builds/184120

[sourcegraph-bot]

Codenotify: Notifying subscribers in CODENOTIFY files for diff cc7bd73...75ff1bf.

Notify	File(s)
@unknwon	dev/gqltest/external_service_test.go dev/gqltest/sub_repo_permissions_test.go

[ryanslade]

Once this is merged we should merge main back into https://github.com/sourcegraph/sourcegraph/pull/44301 and confirm that the new version can also clone a depot.

[ryanslade]

@mrnugget PTAL

[burmudar]

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