Fix test_warn_on_joined_to_many for Ruby 2.7 with Rails 5.2

The test was failing on Ruby 2.7 with Rails 5.2.8.1 because ActiveRecord
deprecation warnings were appearing in stderr alongside the expected
performance warning, causing the exact string match assertion to fail.

Changes:
- Replace assert_equal with assert_match to check for presence of the
  performance warning rather than exact stderr match
- Replace assert_empty with refute_match to verify the performance
  warning doesn't appear when disabled, while tolerating environment-
  specific deprecation warnings

This makes the test more robust across different Ruby/Rails versions
while still validating the core functionality.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: takaokouji

test/controllers/controller_test.rb

@@ -3682,14 +3682,16 @@ def test_warn_on_joined_to_many
       get :index, params: {fields: {posts: 'id,title'}}
       assert_response :success
     end
-    assert_equal(err, "Performance issue detected: `Api::V4::PostResource.records` returned non-normalized results in `Api::V4::PostResource.find_fragments`.\n")
+    assert_match(/Performance issue detected: `Api::V4::PostResource.records` returned non-normalized results in `Api::V4::PostResource.find_fragments`\./, err)
 
     JSONAPI.configuration.warn_on_performance_issues = false
     _out, err = capture_subprocess_io do
       get :index, params: {fields: {posts: 'id,title'}}
       assert_response :success
     end
-    assert_empty err
+    # On older Ruby/Rails combinations, there may be deprecation warnings
+    # but we should not see the performance issue warning
+    refute_match(/Performance issue detected/, err)
 
   ensure
     JSONAPI.configuration = original_config