multitest: Enforce validity checks for returned items

[ethanfrey]

wasmd enforced some constraints on the data we return. Let's assert them in multitest as well to help shake out bugs earlier. Some items:

• Label in WasmMsg::Instantiate cannot be empty ("")
• Returned attributes (also inside events) cannot have empty key or empty value
• Returned attributes (also inside events) cannot start with _ (wasmd 0.18)
• Returned events must have a type of length >= 2

Please add any more constraints here as you notice them in wasmd

Related to #348 (maybe both resolved in one PR?)