tests/gem_concurrent_blit: Fix segmentation fault

tests/gem_concurrent_blit: Fix segmentation fault

Apparently the simple changes introduced in

commit 78865847f9fae7e590960f9836f2ec8b611a190e
Author:     Daniel Vetter <daniel.vetter@ffwll.ch>
AuthorDate: Mon Aug 19 07:23:49 2013 +0200
Commit:     Daniel Vetter <daniel.vetter@ffwll.ch>
CommitDate: Mon Aug 19 08:09:25 2013 +0200

    lib/drmtest: skip fixtures after an igt_skip

pushed the stack space requirement for run_modes over some magic limit
(I guess where the strack grow logic starts to fail, but I didn't
check). Fix this by moving the data into .bss.

Also add some asserts to check that fixtures aren't abused while at
it (i.e. the useful parts of my debug printf/assert craze).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68299
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>