BootLinuxSshTest: Only use 'test' for unittest.TestCase method names

In commit f6e501a2, Eduardo started to use "check_" as a
prefix for methods of similar purpose. Follow this prior art,
since it might become the conventions when writting Avocado
tests.

Suggested-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190607174953.22342-1-philmd@redhat.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>