Fix tsh/common test timeout by sharing kube cluster fixtures

[jakealti]

The tests in tool/tsh/common were timing out in CI.

When a test changes, CI runs it 100 times to check for flakiness, and that job has
a 10 minute limit. TestKube and TestKubeLogin start three Teleport clusters
between them, and they did it on every one of the 100 runs.

Now the clusters are started once and reused. That in turn made every run share
one proxy, which then hit the rate limit on /webapi/ping/:connector, so the
proxy also takes an optional rate limiter config that the fixture sets to empty.
When the field is nil the proxy behaves exactly as before.

The flaky test job used to time out at 10 minutes. It now finishes in 136s.

Each commit stands on its own, so this is easier to review commit by commit.