test: add goleak to detect goroutine leaks

[mafredri]

While doing a quick test/validation for #23, I used goleak to confirm that my suspicion wasn't warranted. Seemed like it would be a useful addition.

Adjusted the test structure slightly to fix the leak.

Before:

goleak: Errors on successful test run: found unexpected goroutines:
[Goroutine 65 in state sync.WaitGroup.Wait, with sync.runtime_SemacquireWaitGroup on top of the stack:
sync.runtime_SemacquireWaitGroup(0xc000302030?)
        /usr/local/go/src/runtime/sema.go:110 +0x25
sync.(*WaitGroup).Wait(0x0?)
        /usr/local/go/src/sync/waitgroup.go:118 +0x48
github.com/coder/quartz.(*Mock).matchCallLocked(0xc00030a000, 0xc0003240c0)
        /home/coder/quartz/mock.go:206 +0x328
github.com/coder/quartz.(*Mock).Now(0xc00030a000, {0x0?, 0xc00017dc00?, 0x5a4770?})
        /home/coder/quartz/mock.go:122 +0x10c
github.com/coder/quartz_test.Test_UnreleasedCalls.func1.1()
        /home/coder/quartz/mock_test.go:404 +0x1e
created by github.com/coder/quartz_test.Test_UnreleasedCalls.func1 in goroutine 16
        /home/coder/quartz/mock_test.go:403 +0xeb
]
exit status 1
FAIL    github.com/coder/quartz 2.460s

Now:

=== RUN   Test_UnreleasedCalls
=== PAUSE Test_UnreleasedCalls
=== CONT  Test_UnreleasedCalls
    mock.go:442: Mock Clock - Trap Now(..., [])
    mock.go:196: Mock Clock - Now([]) call, matched 1 traps
    mock_test.go:408: trap Closed() with 1 unreleased calls
    mock.go:484: Mock Clock - test cleanup; will no longer log clock events
--- PASS: Test_UnreleasedCalls (0.00s)
PASS
ok      github.com/coder/quartz 0.003s

[spikecurtis]

I thought about this, but right now Quartz has zero dependencies, which is appealing for a small test library. I don't think there is a way to tell Go that a library is only for test and doesn't need to be installed when someone imports Quartz.

Should we consider vendoring it?

[mafredri]

That's a good point, I didn't consider it. Vendoring is definitely one option, although I think it'll still show up in go.mod if you go vendor it.

WDYT about a fairly trivial implementation like in a7c6e08 (#24)?