drm/i915/selftests: Use a sacrificial context for hang testing

Avoid injecting hangs in to the i915->kernel_context in case the GPU
reset leaves corruption in the context image in its wake (leading to
continual failures and system hangs after the selftests are ostensibly
complete). Use a sacrificial kernel_context instead.

v2: Closing a context is tricky; export a function (for selftests) from
i915_gem_context.c to get it right.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205152431.12163-2-chris@chris-wilson.co.uk