Previously, we have used `grep -v` to assert something is *not* there.
However, that doesn't work. See for example this file:
$ cat TEST
line1
line2
line3
$ grep -v line4 TEST
line1
line2
line3
$ echo $?
0
This gives a false sense of correctness, however it exits will 0 with anything:
$ grep -v line3 TEST
line1
line2
$ echo $?
0
Instead, we use `! grep` now:
$ ! grep line4 TEST
$ echo $?
0
$ ! grep line3 TEST
line3
$ echo $?
1
Additionally, remove a trailing slash from one of the checks to match both cases
(with or without the slash).
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