Cleanup dependencies

[shuchitak]

Changes:

• Port aec_unit_tests to C. (One of the points covered in issue https://github.com/xmos/sw_avona/issues/103)
• Remove audio_test_tools and lib_dsp as tests dependencies.
• Use https://github.com/ThrowTheSwitch/Unity.git Unity instead of xmos/Unity library and fetch it as part of cmake process. Modified CMakeLists.txt files to fix Unity include paths.