tests: fix some tests for systems with fs.verity.require_signatures=1

tests: fix some tests for systems with fs.verity.require_signatures=1

Some of the newer verity tests don't work properly on systems where
fs.verity.require_signatures is enabled, either because they forget to
disable it at the beginning of the test, or they forget to re-enable it
afterwards, or both.  Fix this.

Reviewed-by: Andrey Albershteyn <aalbersh@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>