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/return/after_else.wren

@@ -0,0 +1,3 @@
+IO.print(new Fn {
+  if (false) "no" else return "ok"
+}.call()) // expect: ok

test/language/return/after_if.wren

@@ -0,0 +1,3 @@
+IO.print(new Fn {
+  if (true) return "ok"
+}.call()) // expect: ok

test/language/return/after_while.wren

@@ -0,0 +1,3 @@
+IO.print(new Fn {
+  while (true) return "ok"
+}.call()) // expect: ok

test/language/return/in_function.wren

@@ -0,0 +1,6 @@
+var f = new Fn {
+  return "ok"
+  IO.print("bad")
+}
+
+IO.print(f.call()) // expect: ok

test/language/return/in_method.wren

@@ -0,0 +1,8 @@
+class Foo {
+  method {
+    return "ok"
+    IO.print("bad")
+  }
+}
+
+IO.print((new Foo).method) // expect: ok

test/language/return/return_null_if_newline.wren

@@ -0,0 +1,6 @@
+var f = new Fn {
+  return
+  IO.print("bad")
+}
+
+IO.print(f.call()) // expect: null