So it can run on targets with low memory, eg esp8266.
Also enable the viper_4args() sub-test, which is now supported.
Signed-off-by: Damien George <damien@micropython.org>
If micropython.native decorator doesn't compile, then we skill all
native/viper tests.
This patch also re-enables the ujson_loads test on NT.
Addresses issue #861, and partially addresses issue #856.