Add doctests for the validate_output_table_type function

[seisman]

Currently, we are testing the behavior of the output_type/outfile parameters in many wrappers' tests (e.g., the 3 grd2xyz tests removed in #3097, filter1d tests in #3085 and grdvolume tests). Since the output_type/outfile parameters are always checked by the validate_output_table_type function, we should test the validate_output_table_type function instead.

The benefits are:

• Few tests to run and faster CI
• We can focus on the core functionality of wrappers rather than worry about the output_type/outfile parameters