testing: Add level argument to ExpectLog

testing: Add level argument to ExpectLog

This makes it possible for tests to be a little more precise, and also
makes them less dependent on exactly how the test is run (runtests.py
sets the logging level to info, but when running tests directly from
an editor it may use the default of warnings-only).

CI only runs the tests with runtests.py, so this might regress, but I'm
not building anything to prevent that yet (options include running the
tests differently in CI or making ExpectLog always use a fixed log
configuration instead of picking up the current one)