Add a "make verify_coverage" target generation coverage info.

Use lcov/gcov to gather coverage info for the tests (Only works with gcc/clang and make).

cmake -DEVENT__COVERAGE=1 -DCMAKE_BUILD_TYPE=Debug ..
make
make verify_coverage

Current coverage (run on debian):
Line coverage 79.1 %	10231 / 12939
Function coverage 86.1 %	933 / 1083