test: make the systemd-run calls synchronous

Otherwise we might be checking results of such calls before they even
finish, causing nasty races like:

```
[   15.656530] testsuite-43.sh[303]: + su testuser -s /bin/sh -c 'XDG_RUNTIME_DIR=/run/user/$UID exec "$@"' -- sh systemd-run --user --unit=test-unprotected-home -P touch /home/testuser/works.txt
...
[   15.757744] testsuite-43.sh[324]: Running as unit: test-unprotected-home.service
[   15.775611] systemd[296]: Started /usr/bin/touch /home/testuser/works.txt.
[   15.783597] testsuite-43.sh[303]: + test -e /home/testuser/works.txt
[   15.787542] systemd[296]: test-unprotected-home.service: Succeeded.
...
[   15.787684] systemd[1]: Received SIGCHLD from PID 303 (bash).
[   15.787790] systemd[1]: Child 303 (bash) died (code=exited, status=1/FAILURE)
[   15.787881] systemd[1]: testsuite-43.service: Child 303 belongs to testsuite-43.service.
[   15.788040] systemd[1]: testsuite-43.service: Main process exited, code=exited, status=1/FAILURE
[   15.788224] systemd[1]: testsuite-43.service: Failed with result 'exit-code'.
[   15.788333] systemd[1]: testsuite-43.service: Service will not restart (restart setting)
[   15.788421] systemd[1]: testsuite-43.service: Changed start -> failed
[   15.788790] systemd[1]: testsuite-43.service: Job 160 testsuite-43.service/start finished, result=failed
[   15.788995] systemd[1]: Failed to start testsuite-43.service.
```

test/units/testsuite-43.sh

@@ -10,10 +10,10 @@ runas() {
     su "$userid" -s /bin/sh -c 'XDG_RUNTIME_DIR=/run/user/$UID exec "$@"' -- sh "$@"
 }
 
-runas testuser systemd-run --user --unit=test-private-users \
+runas testuser systemd-run --wait --user --unit=test-private-users \
     -p PrivateUsers=yes -P echo hello
 
-runas testuser systemd-run --user --unit=test-private-tmp-innerfile \
+runas testuser systemd-run --wait --user --unit=test-private-tmp-innerfile \
     -p PrivateUsers=yes -p PrivateTmp=yes \
     -P touch /tmp/innerfile.txt
 # File should not exist outside the job's tmp directory.
@@ -21,17 +21,17 @@ test ! -e /tmp/innerfile.txt
 
 touch /tmp/outerfile.txt
 # File should not appear in unit's private tmp.
-runas testuser systemd-run --user --unit=test-private-tmp-outerfile \
+runas testuser systemd-run --wait --user --unit=test-private-tmp-outerfile \
     -p PrivateUsers=yes -p PrivateTmp=yes \
     -P test ! -e /tmp/outerfile.txt
 
 # Confirm that creating a file in home works
-runas testuser systemd-run --user --unit=test-unprotected-home \
+runas testuser systemd-run --wait --user --unit=test-unprotected-home \
     -P touch /home/testuser/works.txt
 test -e /home/testuser/works.txt
 
 # Confirm that creating a file in home is blocked under read-only
-runas testuser systemd-run --user --unit=test-protect-home-read-only \
+runas testuser systemd-run --wait --user --unit=test-protect-home-read-only \
     -p PrivateUsers=yes -p ProtectHome=read-only \
     -P bash -c '
         test -e /home/testuser/works.txt
@@ -40,12 +40,12 @@ runas testuser systemd-run --user --unit=test-protect-home-read-only \
 test ! -e /home/testuser/blocked.txt
 
 # Check that tmpfs hides the whole directory
-runas testuser systemd-run --user --unit=test-protect-home-tmpfs \
+runas testuser systemd-run --wait --user --unit=test-protect-home-tmpfs \
     -p PrivateUsers=yes -p ProtectHome=tmpfs \
     -P test ! -e /home/testuser
 
 # Confirm that home, /root, and /run/user are inaccessible under "yes"
-runas testuser systemd-run --user --unit=test-protect-home-yes \
+runas testuser systemd-run --wait --user --unit=test-protect-home-yes \
     -p PrivateUsers=yes -p ProtectHome=yes \
     -P bash -c '
         test "$(stat -c %a /home)" = "0"
@@ -57,7 +57,7 @@ runas testuser systemd-run --user --unit=test-protect-home-yes \
 # namespace (no CAP_SETGID in the parent namespace to write the additional
 # mapping of the user supplied group and thus cannot change groups to an
 # unmapped group ID)
-! runas testuser systemd-run --user --unit=test-group-fail \
+! runas testuser systemd-run --wait --user --unit=test-group-fail \
     -p PrivateUsers=yes -p Group=daemon \
     -P true