kcsan: test: Defer kcsan_test_init() after kunit initialization

When the test is built into the kernel (not a module), kcsan_test_init()
and kunit_init() both use late_initcall(), which means kcsan_test_init()
might see a NULL debugfs_rootdir as parent dentry, resulting in
kcsan_test_init() and kcsan_debugfs_init() both trying to create a
debugfs node named "kcsan" in debugfs root. One of them will show an
error and be unsuccessful.

Defer kcsan_test_init() until we're sure kunit was initialized.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>