test/recipes/30-test_evp.t: Fix multiple definition of @bffiles

Curiously enough, perl only warned about the shadowing.  However, the
following 'plan' statement got disturbed somehow, as one could notice
the test counter say "11/?" instead of "11/25".

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/10944)