tests: add test-execute

add tests for the following directives:
- WorkingDirectory
- Personality
- IgnoreSIGPIPE
- PrivateTmp
- SystemCallFilter: It makes test/TEST-04-SECCOMP obsolete, so it has
  been removed.
- SystemCallErrorNumber
- User
- Group
- Environment

test/exec-environment-multiple.service

@@ -0,0 +1,7 @@
+[Unit]
+Description=Test for Environment
+
+[Service]
+ExecStart=/bin/sh -c 'exit $(test "$VAR1" = "word1 word2") && $(test "$VAR2" = word3) && $(test "$VAR3" = foobar)'
+Environment="VAR1=word1 word2" VAR2=word3 "VAR3=$word 5 6"
+Environment="VAR3=foobar"