vstart_runner.py: add methods for negative testing a cmd

vstart_runner.py: add methods for negative testing a cmd

Methods like run_shell effectively conduct positive test on the given
command. Add methods that runs given command expecting failure and then
verifies return value and error message with given one. Rewrite testcmd,
testcmd_as_user and testcmd_as_root to create these new methods for
negative testing since run_shell, run_as_user and run_as_root is
equivalent of running positive test.

Signed-off-by: Rishabh Dave <ridave@redhat.com>