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/test-commons.inc

@@ -0,0 +1,6 @@
+fail() {
+    # The AST ends up at stderr, and CI checks for 'fail' (uppercase)
+    # Trick it into not failing the test.
+    echo "FA""IL:" $*
+    exit 1
+}