MDEV-20787 Script dgcov.pl does not work

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.

mysql-test/dgcov.pl

@@ -155,14 +155,15 @@ END
 sub gcov_one_file {
   return unless /\.gcda$/;
   unless ($opt_skip_gcov) {
-    $cmd= "gcov -i '$_' 2>/dev/null >/dev/null";
+    $cmd= "gcov -il '$_' 2>/dev/null >/dev/null";
     print STDERR ++$file_no,"\r" if not $opt_verbose and -t STDERR;
     logv "Running: $cmd";
     system($cmd)==0 or die "system($cmd): $? $!";
   }
 
   # now, read the generated file
-  open FH, '<', "$_.gcov" or die "open(<$_.gcov): $!";
+  for my $gcov_file (<$_*.gcov>) {
+    open FH, '<', "$gcov_file" or die "open(<$gcov_file): $!";
     my $fname;
     while (<FH>) {
       chomp;
@@ -175,12 +176,13 @@ sub gcov_one_file {
       }
       next if /^lcount:\d+,-\d+/; # whatever that means
       unless (/^lcount:(\d+),(\d+)/ and $fname) {
-      warn "unknown line '$_' after running '$cmd'";
+        warn "unknown line '$_' in $gcov_file";
         next;
       }
       $cov{$fname}->{$1}+=$2;
     }
     close(FH);
+  }
 }
 
 sub write_coverage {