staging: comedi: Add Kconfig options to build unit test modules

staging: comedi: Add Kconfig options to build unit test modules

The comedi unit-test modules in "drivers/staging/comedi/drivers/tests/"
are built if the `CONFIG_COMEDI_TESTS` option is enabled, but the comedi
Kconfig file contains no code to enable the option.  Add config options
to allow each of the unit-test modules to be enabled individually.  The
"ni_route_tests" module depends on the "ni_routing" module, so select it
if the "ni_route_tests" module is configured to be built.

Cc: Spencer E. Olson <olsonse@umich.edu>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20210407181342.1117754-7-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>