selftests: vm: bring common functions to a new file

Bring common functions to a new file while keeping code as much same as
possible.  These functions can be used in the new tests.  This helps in
avoiding code duplication.

Link: https://lkml.kernel.org/r/20220420084036.4101604-1-usama.anjum@collabora.com


Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Gabriel Krisman Bertazi <krisman@collabora.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>