deps: Upgrade Percy SDK & tweak asset discovery settings

This commit updates the Percy Ember SDK to the latest version. With that updates
the underlying package that does asset discovery (`@percy/agent`). In newer
versions asset discovery response caching is on by default, so we cache all
network requests.

The search svg was missing in random snapshots because the test server was
failing to respond when the asset discovery service would request it to save. By
enabling caching we can reduce the burden on the server and always have that svg
in the cache (along with all other assets that load with a 200).

Author: Robdel12

.percy.yml

@@ -0,0 +1,6 @@
+version: 1
+agent:
+  asset-discovery:
+    network-idle-timeout: 250 # ms
+    page-pool-size-min: 8 # pages
+    page-pool-size-max: 15 # pages