test: add test cases for top-level return restrictions

Add test cases demonstrating that return statements are not allowed at the top-level scope, while being permitted within functions and nested function contexts

Author: ryan-gang

test_programs/r5/1.lox

@@ -1,4 +1,8 @@
 ---
 expected_error_type: compile
 ---
-return "foo";
+fun foo() {
+  return "at function scope is ok";
+}
+
+return; // Error: Can't return from top-level code.

test_programs/r5/2.lox

@@ -1,3 +1,19 @@
 ---
-expected_error_type: none
+expected_error_type: compile
 ---
+fun foo() {
+  if (true) {
+    // Return inside nested blocks in a function is fine
+    return "early return";
+  }
+  
+  for (var i = 0; i < 10; i = i + 1) {
+    // Return inside loops in a function is fine too
+    return "loop return";
+  }
+}
+
+// But this is still an error
+if (true) {
+  return "conditional return"; // Error: Can't return from top-level code.
+}

test_programs/r5/3.lox

@@ -1,3 +1,13 @@
 ---
-expected_error_type: none
+expected_error_type: compile
 ---
+{
+  return "not allowed in a block either"; // Error: Can't return from top-level code.
+}
+
+fun allowed() {
+  if (true) {
+    return "this is fine"; // This is ok
+  }
+  return; // This is also ok
+}

test_programs/r5/4.lox

@@ -1,3 +1,18 @@
 ---
-expected_error_type: none
+expected_error_type: compile
 ---
+fun outer() {
+  fun inner() {
+    return "ok";
+  }
+  
+  return "also ok";
+}
+
+if (true) {
+  fun nested() {
+    return; // This is fine
+  }
+  
+  return "not ok"; // Error: Can't return from top-level code.
+}