MDEV-20787 Script dgcov.pl does not work

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.

mysql-test/dgcov.pl

@@ -171,7 +171,7 @@ sub gcov_one_file {
         next;
       }
       if (/^file:/) {
-        $fname=realpath($');
+        $fname=realpath(-f $' ? $' : $root.$');
         next;
       }
       next if /^lcount:\d+,-\d+/; # whatever that means