Bug #55582 mtr root detection (and if-expression execution) broken

Follow-up patch: added test cases for -0 and while

mysql-test/r/mysqltest.result

@@ -395,6 +395,7 @@ Counter is greater than 0, (counter=10)
 Counter is not 0, (counter=0)
 Counter is true, (counter=alpha)
 Beta is true
+while with string, only once
 1
 Testing while with not
 mysqltest: In included file "MYSQLTEST_VARDIR/tmp/mysqltest_while.inc": At line 64: Nesting too deeply

mysql-test/t/mysqltest.test

@@ -1119,10 +1119,21 @@ if ($counter)
 {
   echo oops, space is true;
 }
+let $counter=-0;
+if ($counter)
+{
+  echo oops, -0 is true;
+}
 if (beta)
 {
   echo Beta is true;
 }
+let $counter=gamma;
+while ($counter)
+{
+  echo while with string, only once;
+  let $counter=000;
+}
 
 # ----------------------------------------------------------------------------
 # Test while, { and }