Set EXE_EXT environment variable when testing

When cross compiling, we may end up with someting like apps/openssl.exe
and a number of test/*.exe.  However, util/shlib_wrap.sh doesn't know
what the executable extension should be, if any, so we need to make
sure it has access to that information when testing, since
OpenSSL::Test uses that script to execute all programs.

Reviewed-by: Rich Salz <rsalz@openssl.org>