For every file.gcda file, gcov <7.x created file.cc.gcda.gcov.
While gcov 7.x and 8.x create file.cc.gcov
And sometimes otherfile.h.gcov or otherfile.ic.gcov, for included files.

(gcov 9.x+ creates .json.gz files, see MDEV-26102)

So, we use `gcov -l` that will create file.cc.gcda##file.cc.gcov,
file.cc.gcda##otherfile.h.gcov, etc. And we search and parse all
those file.cc.gcda*.gcov files.