Use same test names in the default and the TAP13 output format

Use the same test names in the TAP13 output as in the default output
format. This makes the output more consistent. To achieve this, we
need to pass the test name as an argument to the function
`process_test_output`.

Before this change:
$ ./run_tests.sh
PASS selftest-setup (14 tests)
...

vs.

$ ./run_tests.sh -t
TAP version 13
ok 1 - selftest: true
ok 2 - selftest: argc == 3
...

After this change:
$ ./run_tests.sh
PASS selftest-setup (14 tests)
...

vs.

$ ./run_tests.sh -t
TAP version 13
ok 1 - selftest-setup: selftest: true
ok 2 - selftest-setup: selftest: argc == 3
...

While at it, introduce a local variable `kernel` in
`RUNTIME_log_stdout` since this makes the function easier to read.

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Message-Id: <20200825102036.17232-3-mhartmay@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>