[polka]: improve test

Why?

As for now only division operation is checked for correct order of operands (i.e. lhs being on top of rhs).

This patch adds a simple check for subtraction as well.

For instance, my commit with the following synthetic patch applied passes current tests while it should not:

Click here to see the diff

diff --git a/polka/src/lib.rs b/polka/src/lib.rs
index ede8a38..e74bba5 100644
--- a/polka/src/lib.rs
+++ b/polka/src/lib.rs
@@ -49,14 +49,17 @@ impl Interpreter {
                     continue;
                 }
                 "+" | "-" | "*" | "/" => {
-                    let lhs = match self.stk.pop().expect("not enough arguments") {
+                    let mut lhs = match self.stk.pop().expect("not enough arguments") {
                         Value::Number(f) => f,
                         other => panic!("Number expected, got {other}"),
                     };
-                    let rhs = match self.stk.pop().expect("not enough arguments") {
+                    let mut rhs = match self.stk.pop().expect("not enough arguments") {
                         Value::Number(f) => f,
                         other => panic!("Number expected, got {other}"),
                     };
+                    if tok == "-" {
+                        (lhs, rhs) = (rhs, lhs);
+                    }
                     let op = match tok {
                         "+" => f64::add,
                         "-" => f64::sub,