Fix flaky test

[justinvp]

When I was running the tests locally this morning, this was failing because the actual violation message differed from the expected one:

Expected: - Actual: +

- access key must be rotated within 30 days (key is 180 days old)
+ access key must be rotated within 30 days (key is 179 days old)

This change loosens the test slightly to only check for the first part of the message -- the important thing is that we're checking that a violation occurs as expected, which is still the case.

[justinvp]

I'm going to merge this, but happy to address any feedback subsequently.