tests/drmsl: Extract tests out of xf86drmSL.c

v2: merge tests creation and xf86drmSL cleanup
    rename tests/drmsltest -> tests/drmsl
    move the test out of libudev test block
v3: run test even on noudev builds

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>