Just change ENV and restore it afterwards.

Stubbing ENV[] is not safe outside MRI. At some point after the
stubbing has occurred a backtrace is printed to the ActiveSupport
warning log: there Rubinius accesses ENV['RBX_NOCOLOR'] to determine
if it should print the backtrace with colors or not, causing the
stub to fail. Other implementations might access ENV in a different
way too, we just can't predict it.
The only thing we can do here is to actually set the ENV with what
we want and restore it afterwards.

activerecord/test/cases/fixtures_test.rb

@@ -253,7 +253,8 @@ def test_serialized_fixtures
   end
 
   def test_fixtures_are_set_up_with_database_env_variable
-    ENV.stubs(:[]).with("DATABASE_URL").returns("sqlite3:///:memory:")
+    db_url_tmp = ENV['DATABASE_URL']
+    ENV['DATABASE_URL'] = "sqlite3:///:memory:"
     ActiveRecord::Base.stubs(:configurations).returns({})
     test_case = Class.new(ActiveRecord::TestCase) do
       fixtures :accounts
@@ -266,6 +267,8 @@ def test_fixtures
     result = test_case.new(:test_fixtures).run
 
     assert result.passed?, "Expected #{result.name} to pass:\n#{result}"
+  ensure
+    ENV['DATABASE_URL'] = db_url_tmp
   end
 end