Port the main php_cli_server.inc to use ephemeral ports, thus
allowing CLI server tests to be parallelized.
A complication here is that we also need to give each test a
separate doc root, to avoid index.php files writing over each
other.
Closes GH-6375.
1) Prepended starting the server with "exec" so proc_terminate can correctly close it (see also this note http://www.php.net/manual/en/function.proc-get-status.php#93382 for details)
2) Moved putting down the server to the shutdown function to make it independent from tests (fatal) errors
3) Moved php cli executable into the function to make tests more readable
4) changed sleep(1) to usleep(50000) (50 ms) to make tests faster - this needs more testing and if timeout is too small should be increased
