Add `gotestleak` to wsproxy pkg and fix all resulting errors

I tried adding a
```
func TestMain(m *testing.M) {
goleak.VerifyTestMain(m)
}
```
to the `enterprise/coderd/wsproxy` package and there were a ton of failures as a result. We should find some time to figure out where all the leaks are originating from.

Start in the enterprise/coderd/wsproxy package and reproduce the failures from adding TestMain with golek.VerifyTestMain(m). Trace the reported leaks to their sources and consider the work done when the package's leak checks and tests pass without resulting errors.

Vom Indexierungsmodell aus dem Issue-Text verfasst.