remove unittest pkg reference from test_runner

Change-Id: I93d217009ea72c43c1db0ee45b8c2d5fbd9141fe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/140377
Commit-Queue: Kevin Moore <kevmoo@google.com>
Auto-Submit: Kevin Moore <kevmoo@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>

Author: kevmoo

pkg/test_runner/lib/src/utils.dart

@@ -27,8 +27,7 @@ const testPackages = [
   "matcher",
   "meta",
   "path",
-  "stack_trace",
-  "unittest"
+  "stack_trace"
 ];
 
 // TODO(nshahan): Grow this list until it matches the list above. We are