test-execute: simplify checks if grep output is empty

grep already indicates if it matched anything by return value.
Additional advantage is then that if the test fails, the unexpected
matching lines are visible in the log output.

test/test-execute/exec-capabilityboundingset-invert.service

@@ -2,6 +2,6 @@
 Description=Test for CapabilityBoundingSet
 
 [Service]
-ExecStart=/bin/sh -x -c 'c=$$(capsh --print | grep "^Bounding set .*cap_chown"); test -z "$$c"'
+ExecStart=/bin/sh -x -c '! capsh --print | grep "^Bounding set .*cap_chown"'
 Type=oneshot
 CapabilityBoundingSet=~CAP_CHOWN

test/test-execute/exec-privatenetwork-yes.service

@@ -2,6 +2,6 @@
 Description=Test for PrivateNetwork
 
 [Service]
-ExecStart=/bin/sh -x -c 'i=$$(ip link | grep ": " | grep -v ": lo:"); test -z "$$i"'
+ExecStart=/bin/sh -x -c '! ip link | grep ": " | grep -v ": lo:"'
 Type=oneshot
 PrivateNetwork=yes