test/shlibloadtest.c: make some variables block local

myDSO_dsobyaddr and myDSO_free are only used in a narrow block of
code, and can therefore be made local to that block.  Otherwise, some
compilers may warn that they are unused.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5733)