This commit refactors computeExpectedDropReasons to move the core logic
to the utils package. We will need to reuse that logic for another set
of exceptions, this time for XFRM errors.

There are no functional changes in this commit.

Signed-off-by: Paul Chaignon <[email protected]>

We have a couple XFRM errors, XfrmInError and XfrmFwdHdrError, that can
be expected to slightly increase in specific scenarios. Those have
started occuring in CI so we need a way to ignore them.

This commit implements that, as well as a connectivity test flag to
control the list of XFRM errors to ignore. This is similar to what I did
for drop reasons in commit 4880c91 ("connectivity: Check for
unexpected packet drops") and b5e40ba ("connectivity: Add flag
--expected-drop-reasons").

In addition, because those errors are only expected to happen in very
small amount, we only ignore them if there are less than 10 such errors.
It is critical that we fail tests if these errors become more frequent.

Signed-off-by: Paul Chaignon <[email protected]>