Add lcov --initial

Add lcov --initial

By just running lcov on the produced .gcda data files, we don't account
for source files that are not touched by tests at all.  To fix that, run
lcov --initial to create a base line info file with all zero counters,
and merge that with the actual counters when creating the final report.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>