More test/kerberos tweaks

While working on [1] I ended up running into a couple issues with the
Kerberos test suite. Attached are two patches with possible
improvements:

Some tests check for expected log messages. They currently search
through the entire log file, from the beginning, for every match. So if
two tests share the same expected log content (which is common), it's
possible for the second test to get a false positive by matching
against the first test's output. (You can see this by modifying one of
the expected-failure tests to expect the same output as a previous
happy-path test.)

The first patch stores the offset of the previous match, and searches
forward from there during the next match, resetting the offset every
time the log file changes. This isn't perfect -- it could still result
in false positives if one test spits out two or more matching log lines
and only matches the first one by itself -- but searching forward
should be an improvement over what's there now.

The second patch is more of a quality-of-life improvement for devs. On
a failed log match, the test will spin for three minutes before giving
up on the match. I think this is excessive, especially since
interrupting the test with Ctrl-C leaves behind a running KDC daemon.
The patch reduces the timeout to three seconds. I guess the only
question I have is whether there are any underpowered machines out
there running this test, relying on the higher timeout to function.

--Jacob

[1] 
https://www.postgresql.org/message-id/a9ee5e4e8e844d06c2bcf70c6ed3306ccb4897f1.camel%40vmware.com