test/CMakeLists.txt: specifically link libiio to tests
test/CMakeLists.txt: specifically link libiio to tests

In certain conditions/linker configuration, libiio needs to be explicitly
be specified.
Otherwise underlinking won't work too well.

This seems to happen when building libad9361-iio with a different install
prefix.

And from the looks of it, in appveyor we also need to specify this for
stuff to link to tests.

Signed-off-by: Alexandru Ardelean <685557f6b7c225d86b536740313ed51b24843a58@analog.com>

analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio