roachtest: leaked goroutines

Roachtest [prints](https://github.com/cockroachdb/cockroach/blob/2c99066211911e14c7519ec47a5c021488a1fe71/pkg/cmd/roachtest/run.go#L155-L160) leaked goroutines after successful completion.

I'm disabling this in https://github.com/cockroachdb/cockroach/pull/146990 since the leak check has been there for going on two years and few, if any, of the leaks were fixed. Dumping pages of leaked goroutines after each roachtest run is detrimental to the UX of the tool.

TE should re-enable this check only when actually preparing to cut down on the leaked goroutines. Understandably, this hasn't been a priority, and would likely regress unless coupled with some enforcement on a per-test level.

Jira issue: CRDB-51467