Build: Compile QCBOR test suite togheter with the library

QCBOR encode/decode library and its test suite was built
in different places. Move them to the same CMake file
to increase transparency and reduce the include path for
test application.

Change-Id: I645f4e8d4f1e5fb06aa72c00f8fd547a527ba110
Signed-off-by: Tamas Ban <tamas.ban@arm.com>