Default canonifier change to only remove first timestamp in line

In the past, we used a default canonifier, which removes everything that
looks like a timestamp from log files. The goal of this is to prevent
logs from changing, e.g., due to local system times ending up in log
files.

This, however, also has the side-effect of removing information that is
parsed from protocols which probably should be part of our tests.
There is at least one test (1999 certificates) where the entire test
output was essentially removed by the canonifier.

GH-4521 was similarly masked by this.

This commit changes the default canonifier, so that only the first
timestamp in a line is removed. This should skip timestamps that are
likely to change while keeping timestamps that are parsed
from protocol information.

A pass has been made over the tests, with some additional adjustments
for cases which require the old canonifier.

There are some cases in which we probably could go further and not
remove timestamps at all - that, however, seems like a follow-up
project.

testing/scripts/diff-canonifier

@@ -2,4 +2,4 @@
 #
 # Default canonifier used with the tests in testing/btest/*.
 
-$(dirname $0)/diff-remove-timestamps
+$(dirname $0)/diff-remove-first-timestamp