This adds two methods to noui, that allows temporarily suppressing (and
then resuming) the output from noui. For situations where errors are
expected, it's confusing for the test binary to output an error and then
conclude with No errors detected.

It also uses this supress/reconnect in the tests that currently produce
verbose errors when running test_bitcoin.

ninja test_bitcoin
./src/test/test_bitcoin --log_level=message