test: fixing unary operators that are getting parsed as argument instead of string literal (#9951)

* Check for three-string comparison in test

* Add regression tests for unary operator in three-arg form

src/uu/test/src/parser.rs

@@ -188,7 +188,16 @@ impl Parser {
         match symbol {
             Symbol::LParen => self.lparen()?,
             Symbol::Bang => self.bang()?,
-            Symbol::UnaryOp(_) => self.uop(symbol),
+            Symbol::UnaryOp(_) => {
+                // Three-argument string comparison: `-f = a` means "-f" = "a", not file test
+                let is_string_cmp = matches!(self.peek(), Symbol::Op(Operator::String(_)))
+                    && !matches!(Symbol::new(self.tokens.clone().nth(1)), Symbol::None);
+                if is_string_cmp {
+                    self.literal(symbol.into_literal())?;
+                } else {
+                    self.uop(symbol);
+                }
+            }
             Symbol::None => self.stack.push(symbol),
             literal => self.literal(literal)?,
         }

tests/by-util/test_test.rs

@@ -1027,3 +1027,10 @@ fn test_string_lt_gt_operator() {
         .fails_with_code(1)
         .no_output();
 }
+
+#[test]
+fn test_unary_op_as_literal_in_three_arg_form() {
+    // `-f = a` is string comparison "-f" = "a", not file test
+    new_ucmd!().args(&["-f", "=", "a"]).fails_with_code(1);
+    new_ucmd!().args(&["-f", "=", "a", "-o", "b"]).succeeds();
+}