binder: mark binder_alloc_exhaustive_test as slow

binder: mark binder_alloc_exhaustive_test as slow

The binder_alloc_exhaustive_test kunit test takes over 30s to complete
and the kunit framework reports:

  # binder_alloc_exhaustive_test: Test should be marked slow (runtime: 33.842881934s)

Mark the test as suggested to silence the warning.

Cc: Tiffany Yang <ynaffit@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Tiffany Yang <ynaffit@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251024161525.1732874-1-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>