[dart/fuzzer] Use multiple isolates

Change-Id: I8e3882d3a45a3ac8111d2a18c619c7bc20fc4b3d
Reviewed-on: https://dart-review.googlesource.com/c/78161
Reviewed-by: Aart Bik <ajcbik@google.com>
diff --git a/tools/bots/test_matrix.json b/tools/bots/test_matrix.json
index 65b63ecf..b42018b 100644
--- a/tools/bots/test_matrix.json
+++ b/tools/bots/test_matrix.json
@@ -1682,6 +1682,8 @@
           "script": "out/ReleaseX64/dart",
           "arguments": [
             "runtime/tools/dartfuzz/dartfuzz_test.dart",
+            "--isolates",
+            "8",
             "--no-show-stats",
             "--repeat",
             "100"