Add scenario to show problem with goprotobuf package

Unfortunately it's an intermittent failure, so the test isn't reliable
...
Don't bother creating literal values for Mock_XXX types

The whole point of these types is so that you can create a mock object
that satifies a particular interface based on a private type of the
package.  The actual value isn't relevant, so we can just let Go use
the zero value and save ourselves the trouble of trying to figure out
the correct literal syntax.
Add scenario to show how we aren't handling SliceExpr

exprString doesn't know what to do with SliceExpr, which means that we
fail to process code that uses slice expressions in top-level var
statements.
Add code to handle SliceExpr to exprString