Permit tests to specify the refdata filename

Permit tests to specify the refdata filename

When the same test data is re-used for multiple test cases it is
wasteful to serialize that data many times. This functionality permits
test cases to choose their own refdata file name, perhaps derived from
the actual test name. This means the tests can run efficiently, don't
needlessly proliferate lots of data, and express directly in the code
that multiple test cases produce the same data.

Refs #4255