TestPluginBase.transform_format does not validate it output

[Oddant1]

Improvement Description
Validate the output of TestPluginBase.transform_format

Current Behavior
The standard transformation API ensures the validity of the formats output by the transformer., but TestPluginBase.transform_format skips the API and calls the transfomer directly allowing for transformers tested in this manner to potentially produce invalid outputs.

Proposed Behavior
Ensure outputs of transformers called via TestPluginBase.transform_format are valid

References

1. This was first noticed by @ebolyen in the issue IMP: Add transformer from slpssefdf to Casava q2-types#226