va: fix flaky test_http2_http01_challenge int. test. (#4222)
va: fix flaky test_http2_http01_challenge int. test. (#4222)

In some rare conditions the malformed HTTP response error message that
we match in the VA for HTTP-01 connections to HTTP/2 servers will be
returned as a raw `http.badStringError` that doesn't have a transport
connection broken prefix. In these cases the existing
`test_http2_http01_challenge` integration tests fails because the
`h2SettingsFrameErrRegex` doesn't match the returned error.

To accommodate this we make the `h2SettingsFrameErrRegex` optionally
match the transport connection broken prefix.
jcjones/boulder,bmw/boulder,jcjones/boulder,jsha/boulder,jcjones/boulder,bmw/boulder,letsencrypt/boulder,bmw/boulder,jsha/boulder,letsencrypt/boulder,jcjones/boulder,jsha/boulder,letsencrypt/boulder