Cannot filter by labels right after they're added

[foolip]

Steps to reproduce:

• On wpt.fyi, enable "Triage Mode" at the top and go to some directory of tests (any will do)
• Select a few tests and triage them to a label, like "test-label"
• Notice the toast at the bottom that a PR has been created
• Immediately add "filter:test-label" to the search box to filter the tests

The result is that no tests are found. One has to wait for an unknown amount of time before it starts working.

cc @KyleJu

[foolip]

@KyleJu when triaging bugs, they show up in the frontend right away. Could the same be done for labels? And would that fix also work for the search cache, given that's where the filtering happens?