Make OpenSSL::Test::run() sensitive to signals

$? in perl gets the status value from wait(2), which is a word with
the exit code in the upper half and the number of a raised signal in
the lower half.  OpenSSL::Test::run() ignored the signal half up until
now.

With this change, we recalculate an exit code the same way the Unix
shells do, using this formula:

    ($? & 0x7f) ? ($? & 0x7f)|0x80 : ($? >> 8);

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