[CQ] remove stale test_data directory

Change-Id: I4ff42079aa838f7014c2aa011c2fdffb2d5919df
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/403485
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: pq

pkg/linter/analysis_options.yaml

@@ -6,7 +6,6 @@ analyzer:
   exclude:
     # ignore vscode .history files
     - .history/**
-    - test_data/**
   language:
     strict-casts: true
     strict-raw-types: true