Do not call http_do_bio() in isc__nm_http_request()

Do not call http_do_bio() in isc__nm_http_request()

The function should not be called here because it is, in general,
supposed to be called at the end of the transport level callbacks to
perform I/O, and thus, calling it here is clearly a mistake because it
breaks other code expectations. As a result of the call to
http_do_bio() from within isc__nm_http_request() the unit tests were
running slower than expected in some situations.

In this particular situation http_do_bio() is going to be called at
the end of the transport_connect_cb() (initially), or http_readcb(),
sending all of the scheduled requests at once.

This change affects only the test suite because it is the only place
in the codebase where isc__nm_http_request() is used in order to
ensure that the server is able to handle multiple HTTP/2 streams at
once.