[clangd] Improve remote-index test

Introduce a separate thread that will kill `clangd-index-server` after 10 seconds regardless. This helps shut down the test if the server hangs and `stderr.readline()` does not contain inititalizatiton message. It prevents "necessary" waiting delay for the server warm-up and only introduces additional delay if the test fails.

It also makes use of `subprocess.Popen.kill()` which is a portable way of handling process shutdown and avoids using signals.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D90590