renumbers error testers basing on 5000.

fixed a compiler bug not checking the lower bound of block depth in find_variable_backward_with_word()

t/do-5002.err

@@ -0,0 +1,8 @@
+## if `do` is not enclosed in `( )`, it supports only the limited number of expressions.
+
+do ; ## this is ok
+
+do 1; ## this is ok
+
+do { set k 10; printf "k=%d\n" k; } ##ERROR: syntax error - more than one expression after do
+   { set k 20; printf "k=%d\n" k; };