tests: Fix mix-test on big-endian systems

the mix test code never worked on big-endian systems

patch saves a lot of duplicate test and uses more logical naming

see
http://lists.freedesktop.org/archives/pulseaudio-discuss/2014-August/021035.html

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Reported-by: Felipe Sateler <fsateler@debian.org>
util: Fix pa_get_binary_name() on Debian/kFreeBSD

Debian GNU/kFreeBSD uses a FreeBSD kernel and GLIBC,
it #defines __FreeBSD_kernel__, but not __FreeBSD__ nor __linux__
Debian GNU/kFreeBSD does have a /proc/self/exe

FreeBSD #defines __FreeBSD__ and __FreeBSD_kernel__

problem reporte here:
http://lists.freedesktop.org/archives/pulseaudio-discuss/2014-July/020998.html

http://stackoverflow.com/questions/1023306/finding-current-executables-path-without-proc-self-exe and
http://stackoverflow.com/questions/933850/how-to-find-the-location-of-the-executable-in-c have some
background info

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>