tests: Add helper to open a device/module

The new function util_open() encapsulates the standard method employed
by tests to open a device or module. There is a verbatim copy of this in
almost all test programs, with slight variations in the list of modules.
Moving this code into a common helper allows code reuse and makes tests
more consistent.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>