Add more RAPI test utilities

This patch adds a mock RAPI client to test input values to methods. All
methods either raise an exception if there was a problem or return None.
Third-party code can use this to test their input values to the RAPI
client.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>