TestSimpleDebouncer is flakky

[dkropachev]

panic: test timed out after 10m0s
	running tests:
		TestSimpleDebouncer (10m0s)
		TestSimpleDebouncer/Case_2 (10m0s)

goroutine 41 [running]:
testing.(*M).startAlarm.func1()
	/opt/hostedtoolcache/go/1.23.3/x64/src/testing/testing.go:2373 +0x265
created by time.goFunc
	/opt/hostedtoolcache/go/1.23.3/x64/src/time/sleep.go:215 +0x45

goroutine 1 [chan receive]:
testing.(*T).Run(0xc00012a4e0, {0x60bf2c, 0x13}, 0x615578)
	/opt/hostedtoolcache/go/1.23.3/x64/src/testing/testing.go:1751 +0x851
testing.runTests.func1(0xc00012a4e0)
	/opt/hostedtoolcache/go/1.23.3/x64/src/testing/testing.go:2168 +0x86
testing.tRunner(0xc00012a4e0, 0xc00015bae0)
	/opt/hostedtoolcache/go/1.23.3/x64/src/testing/testing.go:1690 +0x[22](https://github.com/scylladb/gocql/actions/runs/11850010539/job/33024189976#step:6:23)7
testing.runTests(0xc000122198, {0x74a320, 0x1, 0x1}, {0x7ff021693a68?, 0x40?, 0x753ea0?})
	/opt/hostedtoolcache/go/1.[23](https://github.com/scylladb/gocql/actions/runs/11850010539/job/33024189976#step:6:24).3/x64/src/testing/testing.go:2166 +0x8bf
testing.(*M).Run(0xc00012c140)
	/opt/hostedtoolcache/go/1.23.3/x64/src/testing/testing.go:2034 +0xf18
main.main()
	_testmain.go:45 +0x165

goroutine 35 [chan receive]:
testing.(*T).Run(0xc00012a680, {0x608cad, 0x6}, 0xc0001[24](https://github.com/scylladb/gocql/actions/runs/11850010539/job/33024189976#step:6:25)7b0)
	/opt/hostedtoolcache/go/1.23.3/x64/src/testing/testing.go:1751 +0x851
github.com/gocql/gocql/debounce.TestSimpleDebouncer(0xc00012a680)
	/home/runner/work/gocql/gocql/debounce/simple_debouncer_test.go:33 +0x4bb
testing.tRunner(0xc00012a680, 0x615578)
	/opt/hostedtoolcache/go/1.23.3/x64/src/testing/testing.go:1690 +0x227
created by testing.(*T).Run in goroutine 1
	/opt/hostedtoolcache/go/1.23.3/x64/src/testing/testing.go:1743 +0x8[26](https://github.com/scylladb/gocql/actions/runs/11850010539/job/33024189976#step:6:27)

goroutine 37 [runnable]:
runtime.Gosched()
	/opt/hostedtoolcache/go/1.23.3/x64/src/runtime/proc.go:353 +0x14
github.com/gocql/gocql/debounce.waitTillChannelIsEmpty(...)
	/home/runner/work/gocql/gocql/debounce/simple_debouncer_test.go:60
github.com/gocql/gocql/debounce.TestSimpleDebouncer.func3(0xc00012a9c0)
	/home/runner/work/gocql/gocql/debounce/simple_debouncer_test.go:48 +0x2bc
testing.tRunner(0xc00012a9c0, 0xc0001247b0)
	/opt/hostedtoolcache/go/1.23.3/x64/src/testing/testing.go:1690 +0x2[27](https://github.com/scylladb/gocql/actions/runs/11850010539/job/33024189976#step:6:28)
created by testing.(*T).Run in goroutine 35
	/opt/hostedtoolcache/go/1.23.3/x64/src/testing/testing.go:1743 +0x826
FAIL	github.com/gocql/gocql/debounce	600.011s
ok  	github.com/gocql/gocql/internal/lru	1.010s
ok  	github.com/gocql/gocql/internal/murmur	1.011s
ok  	github.com/gocql/gocql/internal/streams	1.011s
ok  	github.com/gocql/gocql/internal/tests/serialization	1.010s
ok  	github.com/gocql/gocql/scyllacloud	1.558s
ok  	github.com/gocql/gocql/tests/serialization	20.218s
FAIL

[dkropachev]

@illia-li, could you please take a look into it