patman: Provide a way to intercept commands for testing

Add a test point for the command module. This allows tests to emulate
the execution of commands. This provides more control (since we can make
the fake 'commands' do whatever we like), makes it faster to write tests
since we don't need to set up as much environment, and speeds up test
execution.

Signed-off-by: Simon Glass <sjg@chromium.org>