Fix TCPServer "Should disconnect client" test

[pafuent]

"Should disconnect client" test was failing randomly on Mac CI tests, so this PR is making it more reliable reading on the closed client instead of writing to it.

I run it several times in a loop after fix it to test is not flaky anymore.

for i in `seq 100`; ./bin/godot.macos.editor.dev.x86_64 --test --test-case='[TCPServer] Should disconnect client*'

[demolke]

This should really get merged - the flakiness this is fixing keeps happening to unrelated PRs.

[akien-mga]

From discussing with networking maintainers they're not convinced that this makes the test really reliable / correct testing of TCP.

But let's give it a try as a stopgap solution, and if that doesn't solve the flakiness, we can disable that test.

[akien-mga]

Thanks!