use %! to check $!
Hopefully fix test failures on systems where an unexpected errno is returned

http://stackoverflow.com/questions/7003234/which-systems-define-eagain-and-ewouldblock-as-different-values

e.g.:
http://www.cpantesters.org/cpan/report/01bab316-3069-11e4-89ca-09c8cc4edbc1
http://www.cpantesters.org/cpan/report/0d19010e-3069-11e4-8823-4fcacc4edbc1
Crypt-Random-Source-0.10

          - fix test on older operating systems that define EAGAIN and
            EWOULDBLOCK as different values in errno.h