Added GC stress testing [skip ci]

Author: ankane

test/test_helper.rb

@@ -8,6 +8,23 @@
 require "daru"
 
 class Minitest::Test
+  def setup
+    if stress?
+      # autoload before GC.stress
+      LightGBM::FFI.ffi_libraries
+      load_data
+      GC.stress = true
+    end
+  end
+
+  def teardown
+    GC.stress = false if stress?
+  end
+
+  def stress?
+    ENV["STRESS"]
+  end
+
   def assert_elements_in_delta(expected, actual)
     assert_equal expected.size, actual.size
     expected.zip(actual) do |exp, act|