Move the building of test/buildtest_*. to be done unconditionally

These were guarded by $disabled{tests}.  However, 'tests' is disabled
if we configure 'no-stdio', which means that we don't detect the lack
of OPENSSL_NO_STDIO guards in our public header files.  So we move the
generation and build of test/buildtest_*.c to be unconditional.

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