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/if/if.wren

@@ -0,0 +1,10 @@
+// Evaluate the 'then' expression if the condition is true.
+if (true) IO.print("good") // expect: good
+if (false) IO.print("bad")
+
+// Allow block body.
+if (true) { IO.print("block") } // expect: block
+
+// Assignment in if condition.
+var a = false
+if (a = true) IO.print(a) // expect: true