cpsir unittests: move all unittests into individual files and test runners.

tests/compiler/dart2js/cps_ir/expected/argument_refinement_15.js

+// Expectation for test: 
+// main() {
+//   var x = int.parse('1233');
+//   var y = int.parse('1234');
+//   var z = int.parse('1235');
+//   print(x is num);
+//   print(y is num);
+//   print(z is num);
+//   print(x.clamp(y, z));
+//   print(x is num);
+//   print(y is num);
+//   print(z is num);
+// }
+
+function() {
+  var x = P.int_parse("1233", null, null), y = P.int_parse("1234", null, null), z = P.int_parse("1235", null, null);
+  P.print(typeof x === "number");
+  P.print(typeof y === "number");
+  P.print(typeof z === "number");
+  P.print(J.clamp$2$n(x, y, z));
+  P.print(true);
+  P.print(true);
+  P.print(true);
+}