tests/qtest: Introduce qtest_init_with_env_and_capabilities()

tests/qtest: Introduce qtest_init_with_env_and_capabilities()

This patch adds a new version of qtest_init_with_env() that allows
specifying QMP capabilities that should be enabled during handshake.
This is useful for example if a test needs out-of-band execution of QMP
commands, it can initialize with the oob capability.

Signed-off-by: Juraj Marcin <jmarcin@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>