Merge pull request #26405 from kaingwade:rename_features2d

Rename features2d #26405

This PR renames the module _features2d_ to _features_ as one of the Big OpenCV Cleanup #25007. 
Related PR: opencv/opencv_contrib: [#3820](https://github.com/opencv/opencv_contrib/pull/3820) opencv/ci-gha-workflow: [#192](https://github.com/opencv/ci-gha-workflow/pull/192)

modules/features/test/test_utils.cpp

@@ -0,0 +1,38 @@
+// This file is part of OpenCV project.
+// It is subject to the license terms in the LICENSE file found in the top-level directory
+// of this distribution and at http://opencv.org/license.html.
+#include "test_precomp.hpp"
+
+namespace opencv_test { namespace {
+
+TEST(Features2D_KeypointUtils, retainBest_issue_12594)
+{
+    const size_t N = 9;
+
+    // Construct 4-way tie for 3rd highest - correct answer for "3 best" is 6
+    const float no_problem[] = { 5.0f, 4.0f, 1.0f, 2.0f, 0.0f, 3.0f, 3.0f, 3.0f, 3.0f };
+
+    // Same set, different order that exposes partial sort property of std::nth_element
+    // Note: the problem case may depend on your particular implementation of STL
+    const float problem[] = { 3.0f, 3.0f, 3.0f, 3.0f, 4.0f, 5.0f, 0.0f, 1.0f, 2.0f };
+
+    const size_t NBEST  = 3u;
+    const size_t ANSWER = 6u;
+
+    std::vector<cv::KeyPoint> sorted_cv(N);
+    std::vector<cv::KeyPoint> unsorted_cv(N);
+
+    for (size_t i = 0; i < N; ++i)
+    {
+        sorted_cv[i].response   = no_problem[i];
+        unsorted_cv[i].response = problem[i];
+    }
+
+    cv::KeyPointsFilter::retainBest(sorted_cv, NBEST);
+    cv::KeyPointsFilter::retainBest(unsorted_cv, NBEST);
+
+    EXPECT_EQ(ANSWER, sorted_cv.size());
+    EXPECT_EQ(ANSWER, unsorted_cv.size());
+}
+
+}} // namespace