Make RunCommand less bad

[pbehne]

The sole purpose of the RunCommand tester is to, as the name implies, run an arbitrary command. This capability already exists with the command parameter in the RunApp tester. An investigation should be conducted to assess the feasibility of potential actions to reduce code duplication. Options are:

1. Get ride of RunCommand completely, replace its usage with RunApp.
2. Modify RunApp so it inherits from RunCommand and does not have to implement its own command parameter.
3. ...

If the RunCommand tester is kept, it should be updated to support all options from the class it is derived from (currently Tester). For instance, Tester has the cli_args parameter, but setting it in a RunCommand test has no effect. If a parent parameter cannot be supported, it should be moved to a more derived class or deleted in the class that can't support it.

[pbehne]

Tag @permcody