lib/test_vmalloc.c: introduce xfail for failing tests

lib/test_vmalloc.c: introduce xfail for failing tests

The test align_shift_alloc_test is expected to fail.  Reporting the test
as fail confuses to be a genuine failure.  Introduce widely used xfail
sematics to address the issue.

Note: a warn_alloc dump similar to below is still expected:

 Call Trace:
  <TASK>
  dump_stack_lvl+0x64/0x80
  warn_alloc+0x137/0x1b0
  ? __get_vm_area_node+0x134/0x140

Snippet of dmesg after change:

Summary: random_size_align_alloc_test passed: 1 failed: 0 xfailed: 0 ..
Summary: align_shift_alloc_test passed: 0 failed: 0 xfailed: 1 ..
Summary: pcpu_alloc_test passed: 1 failed: 0 xfailed: 0 ..

Link: https://lkml.kernel.org/r/20250702064319.885-1-raghavendra.kt@amd.com
Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>
Reviewed-by: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: Dev Jain <dev.jain@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>