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 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

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>