selftests/mm: Have the harness run each test category separately

At present the mm selftests are integrated into the kselftest harness by
having it run run_vmtest.sh and letting it pick it's default set of
tests to invoke, rather than by telling the kselftest framework about
each test program individually as is more standard.   This has some
unfortunate interactions with the kselftest harness:

 - If any of the tests hangs the harness will kill the entire mm
   selftests run rather than just the individual test, meaning no
   further tests get run.
 - The timeout applied by the harness is applied to the whole run rather
   than an individual test which frequently leads to the suite not being
   completed in production testing.

Deploy a crude but effective mitigation for these issues by telling the
kselftest framework to run each of the test categories that run_vmtests.sh
has separately.  Since kselftest really wants to run test programs this
is done by providing a trivial wrapper script for each categorty that
invokes run_vmtest.sh, this is not a thing of great elegence but it is
clear and simple.  Since run_vmtests.sh is doing runtime support
detection, scenario enumeration and setup for many of the tests we can't
consistently tell the framework about the individual test programs.

This has the side effect of reordering the tests, hopefully the testing
is not overly sensitive to this.

Signed-off-by: Mark Brown <broonie@kernel.org>