cmd: Add support for watch mode in opa test

Similar to the watch mode available in OPA when run as a server,
this change adds a watch mode in OPA test which
reloads the policy on file-system changes and re-runs the tests.
The watch mode in OPA test could be useful for example in TDD of
policies.

Fixes: #1719

Co-authored-by: Johan Fylling <johan.dev@fylling.se>
Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>