allow specification of tests with pytest -k expression

[fwc]

Currently, enochecker allows test specification like this:

$ enochecker_test -p 1234 -a localhost -A "$IP" test_putflag test_getflag

This PR instead allows to specify tests with a pytest keyword expression, like e.g.

$ enochecker_test -p 1234 -a localhost -A "$IP" "not exploit"

Note that this breaks backwards compatibility for the case in which multiple testcases are specified.