test: properly wait for the forked process

test: properly wait for the forked process

The process forked off by `systemd-notify --fork` is not a child of the
current shell, so using `wait` doesn't work. This then later causes a
race, when the test occasionally fails because it attempts to start a
new systemd-socket-activate instance before the old one is completely
gone:

[ 1488.947744] TEST-74-AUX-UTILS.sh[1938]: Child 1947 died with code 0
[ 1488.947952] TEST-74-AUX-UTILS.sh[1933]: + assert_eq hello hello
[ 1488.949716] TEST-74-AUX-UTILS.sh[1948]: + set +ex
[ 1488.950112] TEST-74-AUX-UTILS.sh[1950]: ++ cat /proc/1938/comm
[ 1488.945555] systemd[1]: Started systemd-networkd.service - Network Management.
[ 1488.950365] TEST-74-AUX-UTILS.sh[1933]: + assert_in systemd-socket systemd-socket-
[ 1488.950563] TEST-74-AUX-UTILS.sh[1951]: + set +ex
[ 1488.950766] TEST-74-AUX-UTILS.sh[1933]: + kill 1938
[ 1488.950766] TEST-74-AUX-UTILS.sh[1933]: + wait 1938
[ 1488.950766] TEST-74-AUX-UTILS.sh[1933]: .//usr/lib/systemd/tests/testdata/units/TEST-74-AUX-UTILS.socket-activate.sh: line 14: wait: pid 1938 is not a child of this shell
[ 1488.950766] TEST-74-AUX-UTILS.sh[1933]: + :
[ 1488.951486] TEST-74-AUX-UTILS.sh[1952]: ++ systemd-notify --fork -- systemd-socket-activate -l 1234 --now socat ACCEPT-FD:3 PIPE
[ 1488.952222] TEST-74-AUX-UTILS.sh[1953]: Failed to listen on [::]:1234: Address already in use
[ 1488.952222] TEST-74-AUX-UTILS.sh[1953]: Failed to open '1234': Address already in use
[ 1488.956831] TEST-74-AUX-UTILS.sh[1933]: + PID=1953
[ 1488.957078] TEST-74-AUX-UTILS.sh[102]: + echo 'Subtest /usr/lib/systemd/tests/testdata/units/TEST-74-AUX-UTILS.socket-activate.sh failed'
[ 1488.957078] TEST-74-AUX-UTILS.sh[102]: Subtest /usr/lib/systemd/tests/testdata/units/TEST-74-AUX-UTILS.socket-activate.sh failed