call GC.compact after each test.

RUBY_TEST_GC_COMPACT=1 enables GC.compact checker which calls
GC.compact after each test.

tool/lib/gc_compact_checker.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+module GCCompactChecker
+  def after_teardown
+    super
+    GC.compact
+  end
+end
+
+Test::Unit::TestCase.include GCCompactChecker if ENV['RUBY_TEST_GC_COMPACT']

tool/test/runner.rb

@@ -9,6 +9,7 @@ require "profile_test_all" if ENV.key?('RUBY_TEST_ALL_PROFILE')
 require "tracepointchecker"
 require "zombie_hunter"
 require "iseq_loader_checker"
+require "gc_compact_checker"
 require_relative "../test-coverage.rb" if ENV.key?('COVERAGE')
 
 case $0