Improve NGA-Sub test coverage

Add minimum number of inputs to hit missing switches.

added GMM label

assigned to @jaltekruse

mentioned in issue #162 (closed)

changed milestone to %Finalize draft NSHM 2023

changed milestone to %Beta release of CONUS NSHM

unassigned @jaltekruse

added 2023 label

Pretty good coverage after recent updates in !338 (merged)
Still some edge cases to work on but should be too time consuming.

changed milestone to %Beta release of CONUS NSHM 2

removed milestone %Beta release of CONUS NSHM 2