tests: Fix *bsd failure in OVS_APP_EXIT_AND_WAIT.

tests: Fix *bsd failure in OVS_APP_EXIT_AND_WAIT.

When using the default user shell in FreeBSD and NetBSD and executing
a test in which the app name parameter to OVS_APP_EXIT_AND_WAIT
contains backticks (e.g. "`pwd`/unixctl"), TMPPID expands to:

    `cat "$OVS_RUNDIR"/"`pwd`"/unixctl.pid 2>/dev/null`

This results in an "unterminated quoted string" syntax error from
the shell.

Correct this by replacing backticks with $(...) in OVS_APP_EXIT_AND_WAIT.
Verified via "make check" under NetBSD 7 and Fedora 23.

Fixes: f9b11f2a09b4 ("tests: Make OVS_APP_EXIT_AND_WAIT() wait for process termination")
Reported-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Lance Richardson <lrichard@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>