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/assignment/syntax.wren

@@ -0,0 +1,11 @@
+// Chained assignment.
+var a = "a"
+var b = "b"
+a = b = "chain"
+IO.print(a) // expect: chain
+IO.print(a) // expect: chain
+
+// Assignment on RHS of variable.
+var c = a = "var"
+IO.print(a) // expect: var
+IO.print(c) // expect: var