Fix flaky TestGc#test_stat_single by disabling GC

test_stat_single fails intermittently:

  1) Failure:
  TestGc#test_stat_single [test/ruby/test_gc.rb:198]:
  <12> expected but was
  <13>.

The test reads the full GC.stat hash and then GC.stat(:count) separately
and asserts they are equal:

    stat = GC.stat
    assert_equal stat[:count], GC.stat(:count)

These are two separate reads of :count. If a GC runs between them, :count
increases and the reads disagree (stat[:count] < GC.stat(:count)). A GC in
that window can be triggered by an allocation on another thread; running
the two reads in a loop with a background allocating thread reproduces it
deterministically (54/2,000,000 mismatches; 0 after the fix).

The exact source of the GC in the failing CI run is unclear (test-all runs
each test process independently), so rather than chase the trigger, disable
GC around the two reads (matching test_stat_heap etc.) so :count cannot
change between them.

CI: https://ci.rvm.jp/results/trunk@ruby-sp3/6395611
log: https://ci.rvm.jp/logfiles/brlog.trunk.20260629-050802

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ko1

test/ruby/test_gc.rb

@@ -194,8 +194,14 @@ def test_stat_argument
   def test_stat_single
     omit 'stress' if GC.stress
 
-    stat = GC.stat
-    assert_equal stat[:count], GC.stat(:count)
+    # GC.stat and GC.stat(:count) are two separate reads of :count. If a GC
+    # runs between them (e.g. triggered by an allocation on another thread),
+    # :count changes and the two reads disagree. Disable GC so both reads
+    # observe the same :count.
+    EnvUtil.without_gc do
+      stat = GC.stat
+      assert_equal stat[:count], GC.stat(:count)
+    end
     assert_raise(ArgumentError){ GC.stat(:invalid) }
   end