staging: comedi: tests: example_test: Reduce stack usage

Declare the `unit_tests` array in `unittest_enter()` `static` to reduce
stack usage a bit.

Cc: Spencer E. Olson <olsonse@umich.edu>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20210407181342.1117754-4-abbotti@mev.co.uk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>