BUG#38817: please make mtr analyze crashes better

Post-push fixes making it work on pushbuild's valgrind host, and clarifying the output.

mysql-test/lib/My/CoreDump.pm

@@ -29,33 +29,33 @@ sub _gdb {
 
   return unless -f $core_name;
 
-  my $dir = tempdir( CLEANUP => 1 );
-  my ($tmp, $tmp_name) = tempfile( DIR => $dir );
+  # Find out name of binary that generated core
+  `gdb -c '$core_name' --batch 2>&1` =~
+    /Core was generated by `([^\s\'\`]+)/;
+  my $binary= $1 or return;
+  print "Core generated by '$binary'\n";
 
+  # Create tempfile containing gdb commands
+  my ($tmp, $tmp_name) = tempfile();
   print $tmp
+    "bt\n",
     "thread apply all bt\n",
     "quit\n";
+  close $tmp or die "Error closing $tmp_name: $!";
 
-  # Find out name of binary that generated core
-  my $list= `gdb -c $core_name -x $tmp_name -batch 2>&1`
-    or return;
-
-  my $binary;
-  foreach my $line (split('\n', $list))
-  {
-    $binary= $1
-      if ($line =~ /Core was generated by `(\S+)/);
-  }
-
-  return unless $binary;
-
-  print " - core generated by '$binary'\n";
+  # Run gdb
+  my $gdb_output=
+    `gdb '$binary' -c '$core_name' -x '$tmp_name' --batch 2>&1`;
 
-  my $list= `gdb $binary -c $core_name -x $tmp_name -batch 2>&1`
-    or return;
+  unlink $tmp_name or die "Error removing $tmp_name: $!";
 
-  print $list, "\n";
+  return unless $gdb_output;
 
+  print <<EOF, $gdb_output, "\n";
+Output from gdb follows. The first stack trace is from the failing thread.
+The following stack traces are from all threads (so the failing one is
+duplicated).
+EOF
   return 1;
 }