refactor: extract validateSuperclass helper and move subclassing tests to inheritance directory

test/inheritance/inherit_from_class.wren

@@ -0,0 +1 @@
+class Subclass is Class {} // expect runtime error: Subclass cannot inherit from Class.

test/inheritance/inherit_from_closure.wren

@@ -0,0 +1,8 @@
+var ClosureType
+
+{
+  var a = "a"
+  ClosureType = new Fn { IO.print(a) }.type
+}
+
+class Subclass is ClosureType {} // expect runtime error: Subclass cannot inherit from Fn.

test/inheritance/inherit_from_fiber.wren

@@ -0,0 +1 @@
+class Subclass is Fiber {} // expect runtime error: Subclass cannot inherit from Fiber.

test/inheritance/inherit_from_fn.wren

@@ -0,0 +1 @@
+class Subclass is Fn {} // expect runtime error: Subclass cannot inherit from Fn.

test/inheritance/inherit_from_list.wren

@@ -0,0 +1 @@
+class Subclass is List {} // expect runtime error: Subclass cannot inherit from List.

test/inheritance/inherit_from_map.wren

@@ -0,0 +1 @@
+class Subclass is Map {} // expect runtime error: Subclass cannot inherit from Map.

test/inheritance/inherit_from_nonclass.wren

@@ -0,0 +1 @@
+class Foo is 123 {} // expect runtime error: Must inherit from a class.

test/inheritance/inherit_from_range.wren

@@ -0,0 +1 @@
+class Subclass is Range {} // expect runtime error: Subclass cannot inherit from Range.

test/inheritance/inherit_from_string.wren

@@ -0,0 +1 @@
+class Subclass is String {} // expect runtime error: Subclass cannot inherit from String.