Shell: Make tests use PASS/FAIL instead of exit codes

There's no guarantee that the last executed command will have a zero
exit code, and so the shell exit code may or may not be zero, even if
all the tests pass.
Also changes the `test || echo fail && exit` to
`if not test { echo fail && exit }`, since that's nicer-looking.

Userland/Shell/Tests/backgrounding.sh

@@ -1,14 +1,12 @@
 #!/bin/Shell
 
 echo "Not running Shell-backgrounding as it has a high failure rate"
+echo PASS
 exit 0
 
-setopt --verbose
+source test-commons.inc
 
-fail(msg) {
-    echo FAIL: $msg
-    exit 1
-}
+setopt --verbose
 
 last_idx=''
 block_idx=0