Provide assertions ivar for Minitest.

Minitest requires an assertion var be available to increment inside
the class that it is running. Add a module that can be mixed in that
provides the methods required.

See minitest/minitest@714cd8a

Author: thomas-holmes

spec/rspec/active_model/mocks/mock_model_spec.rb

@@ -385,6 +385,7 @@
     begin
       require 'minitest/assertions'
       include Minitest::Assertions
+      include MinitestAssertion
     rescue LoadError
       require 'test/unit/assertions'
       include Test::Unit::Assertions

spec/support/minitest_support.rb

@@ -0,0 +1,12 @@
+# Minitest requires you provide an assertions ivar for the context
+# it is running within. We need this so that ActiveModel::Lint tests
+# can successfully be run.
+module MinitestAssertion
+  def assertions
+    @assertions ||= 0
+  end
+
+  def assertions=(value)
+    @assertions = value
+  end
+end