When building with `make` gcov files use full path names,
when building with `ninja` gcov files use paths relative to the source root
in gcov_one_file() the current directory is somewhere under CMakeFiles/,
so if a file exists in the specified location, this location
must've been a full path name.
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.
