package/zlib-ng: disable tests

Tests are enabled by default and can be disabled since
https://github.com/zlib-ng/zlib-ng/commit/b0730d030a9ce5c53faa88bd8259c9afccce6881



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>