package/mraa: disable tests build

mraa build system is looking for a python interpreter >= 2.7 on the host due to
a typo in MRAA_CONF_OPTS. We must use -DBUILDTESTS=OFF instead of -DTESTS=OFF.

Fixes:
http://autobuild.buildroot.net/results/5f0/5f047c4c28b34f6b1bf2306a2b00bcc6a7481c01



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>