Reorganize tests and benchmark scripts.

Mainly to get rid of one top level directory. But this will
also be useful when there are tests of the embedding API.

test/language/this/closure.wren

@@ -0,0 +1,7 @@
+class Foo {
+  getClosure { new Fn { toString } }
+  toString { "Foo" }
+}
+
+var closure = (new Foo).getClosure
+IO.print(closure.call()) // expect: Foo

test/language/this/nested_class.wren

@@ -0,0 +1,22 @@
+class Outer {
+  method {
+    IO.print(this) // expect: Outer
+
+    new Fn {
+      IO.print(this) // expect: Outer
+
+      class Inner {
+        method {
+          IO.print(this) // expect: Inner
+        }
+        toString { "Inner" }
+      }
+
+      (new Inner).method
+    }.call()
+  }
+
+  toString { "Outer" }
+}
+
+(new Outer).method

test/language/this/nested_closure.wren

@@ -0,0 +1,7 @@
+class Foo {
+  getClosure { new Fn { new Fn { new Fn { toString } } } }
+  toString { "Foo" }
+}
+
+var closure = (new Foo).getClosure
+IO.print(closure.call().call().call()) // expect: Foo

test/language/this/this_at_top_level.wren

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

test/language/this/this_in_method.wren

@@ -0,0 +1,6 @@
+class Foo {
+  bar { this }
+  baz { "baz" }
+}
+
+IO.print((new Foo).bar.baz) // expect: baz

test/language/this/this_in_static_method.wren

@@ -0,0 +1,10 @@
+class Foo {
+  static test {
+    IO.print(this == Foo) // expect: true
+    IO.print(this.bar) // expect: bar
+  }
+
+  static bar { "bar" }
+}
+
+Foo.test

test/language/this/this_in_top_level_function.wren

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