selftest/ftrace: Generalise ftracetest to use with RV

selftest/ftrace: Generalise ftracetest to use with RV

The ftracetest script is a fairly complete test framework for tracefs-like
subsystem, but it can only be used for ftrace selftests.

If OPT_TEST_DIR is provided and includes a function file, use that as
test directory going forward rather than just grabbing tests from it.

Generalise function names like initialize_ftrace to initialize_system.

Add the --rv argument to set up the test for rv, basically changing the
trace directory to $TRACING_DIR/rv and displaying an error if that
cannot be found.

This prepares for rv selftests inclusion.

Link: https://lore.kernel.org/r/20251017115203.140080-2-gmonaco@redhat.com
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>