Add label for testing pull requests in CI

.github/workflows/tests.yml

@@ -3,6 +3,8 @@ on:
     types: [submitted]
   push:
   workflow_dispatch:
+  pull_request_target:
+    types: [labeled]
 
 name: Integration Tests
 
@@ -10,6 +12,7 @@ jobs:
   proxy_tests:
     name: Proxy Tests
     runs-on: ubuntu-latest
+    if: github.event_name != 'pull_request' || contains(github.event.pull_request.labels.*.name, 'safe to test')
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1