[gardening] Fix or mark some slow tests.

TEST=ci
Change-Id: Ic453ec3583dd912374f88d53434a1b42ff3f364d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396662
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/tests/vm/vm.status

@@ -35,6 +35,7 @@ dart/slow_path_shared_stub_test: Pass, Slow # Uses --shared-slow-path-triggers-g
 dart/snapshot_version_test: Skip # This test is a Dart1 test (script snapshot)
 dart/spawn_uri_aot_test: Pass, Slow # Runs various subprocesses for testing AOT.
 dart/stack_overflow_shared_test: Pass, Slow # Uses --shared-slow-path-triggers-gc flag.
+dart/timeline_recorder_file_test: Pass, Slow
 
 [ $arch == ia32 ]
 dart/cachable_idempotent_test: Skip # CachableIdempotent calls are not supported in ia32 because it has no object pool.

runtime/vm/heap/heap_test.cc

@@ -737,7 +737,7 @@ ISOLATE_UNIT_TEST_CASE(ArrayTruncationRaces) {
     lists.SetAt(i, list);
   }
 
-  intptr_t kTruncations = 100000;
+  intptr_t kTruncations = 50000;
   for (intptr_t i = 0; i < kTruncations; i++) {
     list ^= lists.At(i % kNumLists);
     array = Array::MakeFixedLength(list);