cmake: create test_interface interface library

Creating a test_interface allows more complex unittest projects to
create additional libraries, for example mock libs, that can be re-used
between test projects.

To support the possibility of such libs to build with same flags as the
testbinary the test_interface is now populated with the public compile
and include flags from testbinary.

testbinary has been updated to use test_interface.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>