selftests/tracing: Test only toplevel README file not the instances

selftests/tracing: Test only toplevel README file not the instances

For the tests that have both a README attribute as well as the instance
flag to run the tests as an instance, the instance version will always
exit with UNSUPPORTED. That's because the instance directory does not
contain a README file. Currently, the tests check for a README file in the
directory that the test runs in and if there's a requirement for something
to be present in the README file, it will not find it, as the instance
directory doesn't have it.

Have the tests check if the current directory is an instance directory,
and if it is, check two directories above the current directory for the
README file:

  /sys/kernel/tracing/README
  /sys/kernel/tracing/instances/foo/../../README

Link: https://lore.kernel.org/r/20250220185846.130216270@goodmis.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>