x86/nmi: Clean up NMI selftest

The expected_testcase_failures variable in the NMI selftest has never
been set since its introduction. Remove this unused variable along with
the related checks to simplify the code.

While at it, replace printk() with the corresponding pr_{cont,info}()
calls. Also, get rid of the superfluous testname wrapper and the
redundant file path comment.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/20250327234629.3953536-9-sohil.mehta@intel.com