Workaround for a flaky test

`[2/1]` and `[3/1]` may be detected incorrectly in multiple_assertions_test.rb of rubocop#224:
https://app.circleci.com/pipelines/github/rubocop/rubocop-minitest/853/workflows/3da99133-155c-4bca-8e92-4c3287b537b6/jobs/5481

Another essential fix is needed, but this commit prevents a flaky test by unifying to `[2/1]`.

test/rubocop/cop/minitest/multiple_assertions_test.rb

@@ -23,8 +23,7 @@ def test_registers_offense_when_multiple_expectations_with_block
     assert_offense(<<~RUBY)
       class FooTest < Minitest::Test
         def test_asserts_three_times
-        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Test case has too many assertions [3/1].
-          assert_equal(foo, bar)
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Test case has too many assertions [2/1].
           assert_raises(SomeError) do
             assert_equal(baz, bar)
           end