[test] Limit running heavy tests sequentially to swarming

This limits the feature of running Memory-hungry tests in sequence to
the GCE bots in swarming. There, the positive memory impact is large
due to the overall lower system memory and the negative runtime impact
is small due to the smaller number of cores.

Bug: v8:11928
Change-Id: Ib577b455d5cccbb3c9855526becbdbf822259bea
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2991630
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Liviu Rau <liviurau@chromium.org>
Reviewed-by: Liviu Rau <liviurau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75406}

tools/testrunner/standard_runner.py

@@ -285,6 +285,10 @@ class StandardTestRunner(base_runner.BaseTestRunner):
     })
     return variables
 
+  def _create_sequence_proc(self, options):
+    """Create processor for sequencing heavy tests on swarming."""
+    return SequenceProc(options.max_heavy_tests) if options.swarming else None
+
   def _do_execute(self, tests, args, options):
     jobs = options.j
 
@@ -309,7 +313,7 @@ class StandardTestRunner(base_runner.BaseTestRunner):
       self._create_predictable_filter(),
       self._create_shard_proc(options),
       self._create_seed_proc(options),
-      SequenceProc(options.max_heavy_tests),
+      self._create_sequence_proc(options),
       sigproc,
     ] + indicators + [
       results,