test: clean up deadlines set in tests

[dfawley]

A straw finally broke the camel's back. Many of our tests don't set timeouts and hang for minutes. This change fixes them, and also standardizes on the use of defaultTest[Short]Timeout everywhere. My goal is to eventually put something like this into vet.sh to make it never happen again:

# Only allow context.Background() to be used with a timeout.
$ find . -name '*_test.go' | xargs grep -n context\.Background | grep -v context.WithTimeout
# Only allow defaultTestTimeout for context.Background() timeouts.
$ find . -name '*_test.go' | xargs grep -n context\.WithTimeout\(context.Background | grep -v defaultTest.*Timeout

There are currently 130 left in our entire tree. Some of these will need to be exempted somehow, or reworked a little to first create a ctx with a timeout and then shorten it from there.

We probably also should look out for selects without a context, bare reads from channels, and any wg.Wait() calls.

RELEASE NOTES: none