test/recipes/01-test_symbol_presence.t: Ignore symbols starting with '__'

test/recipes/01-test_symbol_presence.t: Ignore symbols starting with '__'

On some platforms, the compiler may add symbols that aren't ours and that we
should ignore.

They are generally expected to start with a double underscore, and thereby
easy to detect.

(backport of commit 6c63b7e861819db439551b52ea5594faec04b65c)

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22929)