test: make TEST-27 non-racy

Not sure how I missed this, but we of course need to wait for the
"systemd-run" commands to finish before we can check the output files
this generated.

test/TEST-27-STDOUTFILE/testsuite.sh

@@ -7,23 +7,23 @@ set -o pipefail
 systemd-analyze set-log-level debug
 systemd-analyze set-log-target console
 
-systemd-run --unit=one -p StandardOutput=file:/tmp/stdout -p StandardError=file:/tmp/stderr -p Type=exec sh -c 'echo x ; echo y >&2'
+systemd-run --wait --unit=one -p StandardOutput=file:/tmp/stdout -p StandardError=file:/tmp/stderr -p Type=exec sh -c 'echo x ; echo y >&2'
 cmp /tmp/stdout <<EOF
 x
 EOF
 cmp /tmp/stderr <<EOF
 y
 EOF
 
-systemd-run --unit=two -p StandardOutput=file:/tmp/stdout -p StandardError=file:/tmp/stderr -p Type=exec sh -c 'echo z ; echo a >&2'
+systemd-run --wait --unit=two -p StandardOutput=file:/tmp/stdout -p StandardError=file:/tmp/stderr -p Type=exec sh -c 'echo z ; echo a >&2'
 cmp /tmp/stdout <<EOF
 z
 EOF
 cmp /tmp/stderr <<EOF
 a
 EOF
 
-systemd-run --unit=three -p StandardOutput=append:/tmp/stdout -p StandardError=append:/tmp/stderr -p Type=exec sh -c 'echo b ; echo c >&2'
+systemd-run --wait --unit=three -p StandardOutput=append:/tmp/stdout -p StandardError=append:/tmp/stderr -p Type=exec sh -c 'echo b ; echo c >&2'
 cmp /tmp/stdout <<EOF
 z
 b