MDEV-103: 'debug' is disabled in this build warnings causes tests to fail

The root cause is that after recent fixes around --debug variable (lp:909051)
the variable is now available in both release and debug builds, such that MTR
cannot tell a debug compiled server from optimized one.

To fix, assign a special default value 'disabled' for 'debug' variable in optimized build
and fix MTR to check for this special value to recognize optimized build.

mysql-test/mysql-test-run.pl

@@ -2826,8 +2826,9 @@ sub check_ipv6_support {
 
 sub check_debug_support ($) {
   my $mysqld_variables= shift;
+  my $debug_var= $mysqld_variables->{'debug'};
   
-  if ( ! $mysqld_variables->{'debug'} )
+  if ( !$debug_var || $debug_var eq "disabled")
   {
     #mtr_report(" - binaries are not debug compiled");
     $debug_compiled_binaries= 0;

sql/mysqld.cc

@@ -1182,7 +1182,7 @@ static my_bool plugins_are_initialized= FALSE;
 #ifndef DBUG_OFF
 static const char* default_dbug_option;
 #endif
-static const char *current_dbug_option;
+static const char *current_dbug_option="disabled";
 #ifdef HAVE_LIBWRAP
 const char *libwrapName= NULL;
 int allow_severity = LOG_INFO;