test-systemctl-enable: use magic syntax to allow inverted tests

test-systemctl-enable: use magic syntax to allow inverted tests

Inspired by 7910ec3bcde2ee0086b3e49f8aaa2a9f13f58d97.
'! true' passes, because it's a conditional expression.
But '( ! true )' fails, because '( … )' creates a subshell, i.e. a separate
program, and '! true' becomes the return value of that program, and the whole
thing apparently is not a conditional expression for the outer shell.

This is shorter, so let's just do this.