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/else.wren

@@ -0,0 +1,6 @@
+// Evaluate the 'else' expression if the condition is false.
+if (true) IO.print("good") else IO.print("bad") // expect: good
+if (false) IO.print("bad") else IO.print("good") // expect: good
+
+// Allow block body.
+if (false) null else { IO.print("block") } // expect: block