selftests/mm: log run_vmtests.sh results in TAP format

When running tests on a CI system (e.g.  LAVA) it is useful to output test
results in TAP (Test Anything Protocol) format so that the CI can parse
the fine-grained results to show regressions.  Many of the mm selftest
binaries already output using the TAP format.  And the kselftests runner
(run_kselftest.sh) also uses the format.  CI systems such as LAVA can
already handle nested TAP reports.  However, with the mm selftests we have
3 levels of nesting (run_kselftest.sh -> run_vmtests.sh -> individual test
binaries) and the middle level did not previously support TAP, which
breaks the parser.

Let's fix that by teaching run_vmtests.sh to output using the TAP format. 
Ideally this would be opt-in via a command line argument to avoid the
possibility of breaking anyone's existing scripts that might scrape the
output.  However, it is not possible to pass arguments to tests invoked
via run_kselftest.sh.  So I've implemented an opt-out option (-n), which
will revert to the existing output format.

Future changes to this file should be aware of 2 new conventions:

 - output that is part of the TAP reporting is piped through tap_output
 - general output is piped through tap_prefix

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ryan Roberts <[email protected]>
Reviewed-by: Mark Brown <[email protected]>
Tested-by: John Hubbard <[email protected]>
Cc: Aishwarya TCV <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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