BUG#42485: Pushbuild2 and some build tools do not work together with MTR2

It seems that the length of the thick line printed by mtr when printing the 
suite name differs from mtr1 and mtr2, affecting the mtr filtering by PB2.
      
This patch addresses it by restoring the thick line length to 78 (original
length) instead of 60 (the one in mtr2).

mysql-test/lib/mtr_report.pm

@@ -354,7 +354,7 @@ sub mtr_print_line () {
 
 sub mtr_print_thick_line {
   my $char= shift || '=';
-  print $char x 60, "\n";
+  print $char x 78, "\n";
 }