tests: Add fixtures.h with common constants and other global stuff.
tests: prefix for C test changed to 'test_'.
locate_metadata: Add error check for locating metadata call.
utils: Better scripts for generating tarballs and rpms.
tests: test_data/ renamed to testdata/.
CMake: Remove unused CPACK stuff.
Update README.md
tests: Better run_gtester.sh script.
tests: exclude downloading tests

If this tests were enabled, it would cause a trouble during building package in the Koji.
Downloading is from Internet is not allowed in Koji, so this tests were failing during a build.
(Note: Failed tests means that build will be unsucessful!)