[dartfuzz] Fix flag fuzzer to account for removed serial scavenge and marking.

TEST=dartfuzz
Bug: https://github.com/dart-lang/sdk/issues/60681
Change-Id: I469123c6bacb1c1c084286951022ffe8bfbeddc3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/426981
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/tools/dartfuzz/flag_fuzzer.dart

@@ -39,13 +39,13 @@ final gcFlags = [
   "--dontneed_on_sweep",
   "--force_evacuation",
   "--mark_when_idle",
-  "--marker_tasks=${range(0, 3)}", // 0 disables parallel marking
+  "--marker_tasks=${range(1, 3)}",
   "--no_concurrent_mark",
   "--no_concurrent_sweep",
   "--no_inline_alloc",
   "--runtime_allocate_old",
   "--runtime_allocate_spill_tlab",
-  "--scavenger_tasks=${range(0, 3)}", // 0 disables parallel scavenge
+  "--scavenger_tasks=${range(1, 3)}",
   "--use_compactor",
   "--verify_after_gc",
   "--verify_after_marking",