[recipes] replace the variable ${platform} with resolved-platform in test output

The intention is to create parity with cipd module[1] in recipe engine so that
there will be less friction when downstream repo starts to consume cipd module in
recipe engine

[1]: https://chromium.googlesource.com/infra/luci/recipes-py/+/refs/heads/master/recipe_modules/cipd/test_api.py#23

R=iannucci

Bug: 875523
Change-Id: I5f9100a81e7ca3c7300ab3b0ba198120d598ca48
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Recipe-Nontrivial-Roll: release_scripts
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1951224
Auto-Submit: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

recipes/recipe_modules/cipd/test_api.py

@@ -19,7 +19,7 @@ class CIPDTestApi(recipe_test_api.RecipeTestApi):
 
   def make_pin(self, package_name, version=None):
     return {
-        'package': package_name,
+        'package': package_name.replace('${platform}', 'resolved-platform'),
         'instance_id': self.make_resolved_version(version),
     }