1. Replace isFinished atomic variable with a channel, no functional changes
here, just use more common way as all our services do.
2. Do not check erors from SetReadDeadline and ReadMessage on exit. It
seems to be not quite right because connection is not closed by this
moment, and thus, these error checks are racy.
3. Add read timeout for the message reader. It is needed because some
tests may leave message unread in the end which results in hanging
test cleanup.
4. Add drain loop to message reader in order not to block WS reader on
sending message.
Ref. https://github.com/nspcc-dev/neo-go/pull/3392#issuecomment-2031590403.
It's clear that TestBadSubUnsub is hanging on test cleanup, in
particular, on attempt to wait for wsRead routine to exit. The only
place where wsRead routine may hang is sending to msgCh in case if
receiver is not going to read from this channel:
```
2024-04-02T08:14:51.4957621Z goroutine 14329 [chan receive]:
2024-04-02T08:14:51.4958010Z github.com/nspcc-dev/neo-go/pkg/services/rpcsrv.initCleanServerAndWSClient.func1()
2024-04-02T08:14:51.4958344Z D:/a/neo-go/neo-go/pkg/services/rpcsrv/subscription_test.go:80 +0x71
2024-04-02T08:14:51.4958457Z testing.(*common).Cleanup.func1()
2024-04-02T08:14:51.4958757Z C:/hostedtoolcache/windows/go/1.22.1/x64/src/testing/testing.go:1175 +0x17a
2024-04-02T08:14:51.4958903Z testing.(*common).runCleanup(0xc002cf5860, 0x0)
2024-04-02T08:14:51.4959193Z C:/hostedtoolcache/windows/go/1.22.1/x64/src/testing/testing.go:1353 +0x262
2024-04-02T08:14:51.4959291Z testing.tRunner.func2()
2024-04-02T08:14:51.4959566Z C:/hostedtoolcache/windows/go/1.22.1/x64/src/testing/testing.go:1683 +0x51
2024-04-02T08:14:51.4959695Z testing.tRunner(0xc002cf5860, 0x141687410)
2024-04-02T08:14:51.4959976Z C:/hostedtoolcache/windows/go/1.22.1/x64/src/testing/testing.go:1695 +0x25e
2024-04-02T08:14:51.4960115Z created by testing.(*T).Run in goroutine 1
2024-04-02T08:14:51.4960385Z C:/hostedtoolcache/windows/go/1.22.1/x64/src/testing/testing.go:1742 +0x826
```
Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>
29deba45ca