dma-buf: Introduce selftesting framework

In light of recent review slip ups, the absence of a suite of tests for
dma-buf became apparent. Given the current plethora of testing
frameworks, opt for one already in use by Intel's CI and so allow easy
hook up into igt.

We introduce a new module that when loaded will execute the list of
selftests and their subtest. The names of the selftests are put into the
modinfo as parameters so that igt can identify each, and run them
independently, principally for ease of error reporting.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819095928.32091-1-chris@chris-wilson.co.uk