selftests: vm: refactor run_vmtests.sh to reduce boilerplate

Previously, each test printed out its own header, dealt with its own
return code, etc.  By just putting this standard stuff in a function, we
can delete > 300 lines from the script.

This also makes adding future tests easier. And, it gets rid of various
inconsistencies that already exist:

- Some tests correctly deal with ksft_skip, but others don't.
- Some tests just print the executable name, others print arguments, and
  yet others print some comment in the header.
- Most tests print out a header with two separator lines, but not the
  HMM smoke test or the memfd_secret test, which only print one.
- We had a redundant "exit" at the end, with all the boilerplate it's an
  easy oversight.

Link: https://lkml.kernel.org/r/20220421224928.1848230-1-axelrasmussen@google.comSigned-off-by: Axel Rasmussen <axelrasmussen@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>