mediastreamer: disable test programs

Building tests produces an error during the "OBJCLD mediastream" phase with
some toolchains/architectures.

This is supposed to fix:
 - http://autobuild.buildroot.net/results/7fca436524e5f7ba300d27c615061b0838adb1e2
 - http://autobuild.buildroot.net/results/bd9027ce94348025f1ca02a77eb81b9200124d9a
 - http://autobuild.buildroot.net/results/4d9f554fc29901be1a577ca0c39369befee269b5



Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>