Remediate a number go-routine leaks (mainly test issues)

[k-wall]

• NewClusterAdmin would leak a Client if Controller could not be found
• unclosed MockBroker and Client leaks in unit tests

[dnwe]

Thanks, these are great.

Out of interest did you catch them just through observing the leak during test runs, or by a linter or leak detector?

[k-wall]

it was the test failure under #2197 that prompted me to look.

I found golang/go#6705 which lead me to https://github.com/uber-go/goleak. I added a temporary TestMain added into the tests and then this approach to actually find those that leaked.

func TestMain(m *testing.M) {
	goleak.VerifyTestMain(m, goleak.IgnoreTopFunction("github.com/rcrowley/go-metrics.(*meterArbiter).tick"))
}

Having something built into the build to find goroute leaks/fail the build would be best, but this would require more work.

There were some goroutine leaks remaining that didn't have an obvious cause. I've raised a separate PR showing those remaining leaks and how I detected them.