twister: harness: introduce shell harness

Introduce a new harness based on pytest that does basic shell command
handling. The harness is enabeld using:

harness: shell

and expects a file with parameters in the form:

test_shell_harness:
- command: "kernel version"
  expected: "Zephyr version .*"
- ...

Multiple commands and their expected output can be tested.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>