test: fix tarantool process teardown

Test uses a popen module that starts tarantool process in background
mode. Tarantool process started in background mode forks a new process
and closes a parent, after that popen loses a PID of the started process
and `ph:kill()` and `ph:terminate()` doesn't work anymore. It leads to
non-terminated tarantool processes after running the test.

Patch fixes that by running `kill` using os.execute with a PID of
tarantool process written to a pid file.

Follows up #6128

NO_CHANGELOG=fix test
NO_DOC=fix test

(cherry picked from commit 88686227)