Feature: Add support for generic checks in running config.

[chetryan]

As an engineer writing custom test catalogs, I would find it useful to be able to check that certain lines are in the running config, especially if there are no existing tests for the functionality I am interested in.

Example:

• Checking that the enable password is set
• Checking for a particular username has been set in the config.

Probably would need to support a regex search or searching for line that include the given text.