tests: use relative paths in ExecStart= and friends

We want to retain *some* of the full paths in order to test more code paths.
But the default should be to use the command name only. This makes the tests
less visually cluttered.

test/test-execute/exec-cpuaffinity2.service

@@ -3,7 +3,7 @@
 Description=Test for CPUAffinity (reset)
 
 [Service]
-ExecStart=/bin/sh -c 'test $$(cat /proc/self/status | grep Cpus_allowed: | rev | cut -c 1) = 1'
+ExecStart=sh -c 'test $$(cat /proc/self/status | grep Cpus_allowed: | rev | cut -c 1) = 1'
 CPUAffinity=0-1 3
 CPUAffinity=
 CPUAffinity=0