selftest/vm: add skip support to mremap_test

Allow the mremap test to be skipped due to errors such as failing to
parse the mmap_min_addr sysctl.

Link: https://lkml.kernel.org/r/20220420215721.4868-4-sidhartha.kumar@oracle.com


Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>