openssl: disable tests unless ptest is enabled

openssl: disable tests unless ptest is enabled

The upstream Makefile always builds the tests unless they're explicitly
disabled. Whilst this doesn't make a difference to the final package and
sysroot output, disabling the tests for openssl-native reduces the size
of the build tree from 659M to 78M and reduces the CPU time used by 30%.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>