EAMxx: add testmod to cover diagnostics outputs, including composability and fillval treatment

[mahf708]

Currently, we only rigorously test diagnostics in their isolated standalone testing; we should add a testmod that will cover the following:

1. a representative set of all possible diagnostics outputs
2. a representative set of supported composable diagnostics outputs
3. a representative set of fillval treatment on diagnostics

xref #6935 xref #6803