The current "snap create-user" test runs on classic only. This
is bad for multiple reasons. The main use-case of this feature
are core devices and we are considering to drop support for this
on classic entirely.
testutil: make mocked command work with shellcheck from snaps
When shellcheck is running from a snap it will get a private /tmp. Scripts are
generated in a workdir that is places under /tmp, but inside the host mount
namespace, thus the file will not be accessible by shellcheck.
Work around that by feeding the script to be checked through stdin.
Signed-off-by: Maciej Borzecki <email address hidden>