tests: Fix issue in use of OVS_APP_EXIT_AND_WAIT.

tests: Fix issue in use of OVS_APP_EXIT_AND_WAIT.

Commit f9b11f2a09b4 introduced a loop to wait for process exit
in OVS_APP_EXIT_AND_WAIT after the "exit" command has been sent.
Unfortunately, this does not work for cases where a unixctl socket
has to be used to send the "exit" command because the process
ID cannot be determined from the socket path.

OVS_APP_EXIT_AND_WAIT_BY_TARGET has since been introduced to enable
graceful termination of daemons via unixctl sockets.

This set of changes addresses the problem described above by
making OVS_APP_EXIT_AND_WAIT_BY_TARGET take the unixctl socket
path and corresponding process ID as separate parameters. In order
to better detect issues in this logic in the future, checks have
been added to verify that the pidfile exists before using its
contents.

Tested on a Linux system.

Signed-off-by: Lance Richardson <lrichard@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>