tests/amdgpu: add amdgpu_vram_frag test

BO allocations can take very long time on a fragmented
address space. This test will introduce fragmentation and wait
for 100k bo allocation to be done in 5 seconds. Kernel with a
recent[1] fix should be able to finish this test in < 5 sec.

Output after this patch with the kernel fix:

./tests/amdgpu/amdgpu_test -s 2
<snip>
Suite: BO Tests
  Test: Export/Import ...passed
  Test: CPU map/unmap ...passed
  Test: Memory alloc Test ...passed
  Test: Memory fail alloc Test ...passed
  Test: VRAM Fragmentation Test ...
	amdgpu_vram_frag: Total time taken 0.475837 secs
	passed
  Test: Find bo by CPU mapping ...passed
<snip>

Output after this patch without the kernel fix:
./tests/amdgpu/amdgpu_test -s 2
<snip>
Suite: BO Tests
  Test: Export/Import ...passed
  Test: CPU map/unmap ...passed
  Test: Memory alloc Test ...passed
  Test: Memory fail alloc Test ...passed
  Test: VRAM Fragmentation Test ... timed out
	FAILED
    1. ../tests/amdgpu/bo_tests.c:366  - CU_ASSERT_EQUAL(r,0)
  Test: Find bo by CPU mapping ...passed
 <snip>

[1] https://cgit.freedesktop.org/drm/drm/commit/?id=0cdea4455acd350a7f62406478e3d6d1f764cef9
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
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