drm/xe: Add a xe_bo subtest for shrinking / swapping

Add a subtest that tries to allocate twice the amount of
buffer object memory available, write data to it and then read
all the data back verifying data integrity.
In order to be able to do this on systems that
have no or not enough swap-space available, allocate some memory
as purgeable, and introduce a function to purge such memory from
the TTM swap_notify path.

this test is intended to add test coverage to the current
bo swap path and upcoming shrinking path.

The test has previously been part of the xe bo shrinker series.

v2:
- Skip test if the execution time is expected to be too long.
- Minor code cleanups.

v3:
- Print random seed. (Matthew Auld)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240909085654.5064-1-thomas.hellstrom@linux.intel.com