Avoid race in diode.Close with waiter

[ddn0]

In about 5% of instances of

go test -count 100000 -run TestClose -timeout 30s .

the test ends in a deadlock:

panic: test timed out after 30s

goroutine 6465 [running]:
testing.(*M).startAlarm.func1()
        /home/ddn/.conda/envs/katana-dev/go/src/testing/testing.go:2029 +0x8e
created by time.goFunc
        /home/ddn/.conda/envs/katana-dev/go/src/time/sleep.go:176 +0x32

goroutine 1 [chan receive]:
testing.(*T).Run(0xc000636000, {0x54a7e4?, 0x4bba45?}, 0x555b78)
        /home/ddn/.conda/envs/katana-dev/go/src/testing/testing.go:1487 +0x37a
testing.runTests.func1(0xc000636000?)
        /home/ddn/.conda/envs/katana-dev/go/src/testing/testing.go:1839 +0x6e
testing.tRunner(0xc000636000, 0xc000069cd8)
        /home/ddn/.conda/envs/katana-dev/go/src/testing/testing.go:1439 +0x102
testing.runTests(0xc0000a63c0?, {0x63b0e0, 0x2, 0x2}, {0x7fb6ff136108?, 0x40?, 0x643c00?})
        /home/ddn/.conda/envs/katana-dev/go/src/testing/testing.go:1837 +0x457
testing.(*M).Run(0xc0000a63c0)
        /home/ddn/.conda/envs/katana-dev/go/src/testing/testing.go:1719 +0x5d9
main.main()
        _testmain.go:55 +0x1aa

goroutine 27226 [sync.Cond.Wait]:
sync.runtime_notifyListWait(0xc00063a090, 0x0)
        /home/ddn/.conda/envs/katana-dev/go/src/runtime/sema.go:513 +0x13d
sync.(*Cond).Wait(0xc00063a080?)
        /home/ddn/.conda/envs/katana-dev/go/src/sync/cond.go:56 +0x8c
github.com/rs/zerolog/diode/internal/diodes.(*Waiter).Next(0xc0004ee090)
        /home/ddn/w/zerolog/diode/internal/diodes/waiter.go:69 +0x9b
github.com/rs/zerolog/diode.Writer.poll({{0x57a600, 0xc0004ee060}, {0x57ae68, 0xc0004ee090}, 0xc000626040, 0xc000602120})
        /home/ddn/w/zerolog/diode/diode.go:96 +0x83
created by github.com/rs/zerolog/diode.NewWriter
        /home/ddn/w/zerolog/diode/diode.go:70 +0x36f

goroutine 27224 [chan receive]:
github.com/rs/zerolog/diode.Writer.Close({{0x57a600, 0xc0004ee060}, {0x57ae68, 0xc0004ee090}, 0xc000626040, 0xc000602120})
        /home/ddn/w/zerolog/diode/diode.go:86 +0x4a
github.com/rs/zerolog/diode_test.TestClose(0xc0006e6a00?)
        /home/ddn/w/zerolog/diode/diode_test.go:38 +0x1eb
testing.tRunner(0xc0006361a0, 0x555b78)
        /home/ddn/.conda/envs/katana-dev/go/src/testing/testing.go:1439 +0x102
created by testing.(*T).Run
        /home/ddn/.conda/envs/katana-dev/go/src/testing/testing.go:1486 +0x35f
FAIL	github.com/rs/zerolog/diode	30.008s

This is due to a missed wakeup on Close. Avoid this by holding a mutex on Broadcast, which is not necessary in general but is necessary here.

[ddn0]

I tried to use the existing test TestNewWriter rather than introducing TestClose but it is flaky under frequent execution.

I get errors like

--- FAIL: TestNewWriter (0.00s)
    diode_test.go:29: Diode New Writer Test failed. got:, want:{"level":"debug","message":"test"}
        !

To highlight the specific bug that led me to this fix, I decided to write a separate unit test.