Ignore thread leaks from Dart VM in tsan instrumented builds. (#13049)

diff --git a/testing/tsan_suppressions.txt b/testing/tsan_suppressions.txt
index 6d09bbe..dd4b715 100644
--- a/testing/tsan_suppressions.txt
+++ b/testing/tsan_suppressions.txt
@@ -3,6 +3,7 @@
 
 # We don't care about errors from the Dart VM.
 race:dart::*
+thread:dart::*
 
 # Data race on animator begin frame when the engine is shutting down.
 # https://github.com/flutter/flutter/issues/42190