[infra] Mark run_abi_tests.py as a custom test runner

This will cause the build recipe to process any results.json files this
step produces.

Change-Id: I0ae34a9373bffd5a3757919d5329927571e92925
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106342
Auto-Submit: Alexander Thomas <athom@google.com>
Reviewed-by: Liam Appelbe <liama@google.com>
Commit-Queue: Liam Appelbe <liama@google.com>
diff --git a/tools/bots/test_matrix.json b/tools/bots/test_matrix.json
index 159d812..1f91c88 100644
--- a/tools/bots/test_matrix.json
+++ b/tools/bots/test_matrix.json
@@ -2111,7 +2111,8 @@
         },
         {
           "name": "run tests",
-          "script": "tools/run_abi_tests.py"
+          "script": "tools/run_abi_tests.py",
+          "testRunner": true
         }
       ]
     }