snappy: disable tests

This will also fix an unsafe header path problem:

/br/output/host/usr/bin/mips-linux-gnu-g++ -DHAVE_CONFIG_H -I.
-I/usr/include -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE
-D_FILE_OFFSET_BITS=64  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE
-D_FILE_OFFSET_BITS=64   -Os  -c -o snappy_unittest-snappy-test.o `test
-f 'snappy-test.cc' || echo './'`snappy-test.cc
mips-linux-gnu-g++: ERROR: unsafe header/library path used in
cross-compilation: '/usr/include'
Makefile:730: recipe for target 'snappy_unittest-snappy_unittest.o'
failed

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>