Reorganize tests.

test/this/this_at_top_level.wren

@@ -0,0 +1 @@
+this // expect error

test/this/this_in_method.wren

@@ -0,0 +1,6 @@
+class Foo {
+  bar { this }
+  baz { "baz" }
+}
+
+Foo.new.bar.baz

test/this/this_in_top_level_function.wren

@@ -0,0 +1,3 @@
+fn {
+  this // expect error
+}