Don't assume to know the shared library extension

test/shlibloadtest.c assumes all Unix style platforms use .so as
shared library extension.  This is not the case for Mac OS X, which
uses .dylib.  Instead of this, have the test recipe find out the
extension from configuration data.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1844)