Refactor the way that TestApi works so that it is actually useful.

scripts/slave/recipe_modules/platform/test_api.py

+from slave import recipe_test_api
+
+class PlatformTestApi(recipe_test_api.RecipeTestApi):
+  @recipe_test_api.mod_test_data
+  @staticmethod
+  def name(name):
+    assert name in ('win', 'linux', 'mac')
+    return name
+
+  @recipe_test_api.mod_test_data
+  @staticmethod
+  def bits(bits):
+    assert bits in (32, 64)
+    return bits
+
+  def __call__(self, name, bits):
+    return self.name(name) + self.bits(bits)
+