torture: Add testing of RCU's Rust bindings to torture.sh

This commit adds a --do-rcu-rust parameter to torture.sh, which invokes
a rust_doctests_kernel kunit run.  Note that kunit wants a clean source
tree, so this runs "make mrproper", which might come as a surprise to
some users.  Should there be a --mrproper parameter to torture.sh to make
the user explicitly ask for it?

Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>