check GC.enable'd status

Check GC.enable'd status before and after test execution.

Write this checker in gc_checker.rb, it was renamed from
gc_compact_checker.rb.

tool/lib/gc_checker.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module GCDisabledChecker
+  def before_setup
+    if @__gc_disabled__ = GC.enable # return true if GC is disabled
+      GC.disable
+    end
+
+    super
+  end
+
+  def after_teardown
+    super
+
+    disabled = GC.enable
+    GC.disable if @__gc_disabled__
+
+    if @__gc_disabled__ != disabled
+      label = {
+        true => 'disabled',
+        false => 'enabled',
+      }
+      raise "GC was #{label[@__gc_disabled__]}, but is #{label[disabled]} after the test."
+    end
+  end
+end
+
+module GCCompactChecker
+  def after_teardown
+    super
+    GC.compact
+  end
+end
+
+Test::Unit::TestCase.include GCDisabledChecker
+Test::Unit::TestCase.include GCCompactChecker if ENV['RUBY_TEST_GC_COMPACT']

tool/lib/test/unit/parallel.rb

@@ -6,7 +6,7 @@
 require "tracepointchecker"
 require "zombie_hunter"
 require "iseq_loader_checker"
-require "gc_compact_checker"
+require "gc_checker"
 
 module Test
   module Unit

tool/test/runner.rb

@@ -9,7 +9,7 @@
 require "tracepointchecker"
 require "zombie_hunter"
 require "iseq_loader_checker"
-require "gc_compact_checker"
+require "gc_checker"
 require_relative "../test-coverage.rb" if ENV.key?('COVERAGE')
 
 case $0