Fixes#28095
Problem:
- Stateful kernels dereference null state pointer on line 446 in gcpukernel.hpp
- jinboson confirmed state_ptr is null on x86 Ubuntu (7 hours ago)
- Causes crash with std::shared_ptr assertion on LoongArch64 and strict platforms
Solution (addressing Copilot review feedback from PR #28096):
1. Added null pointer check using CV_Error instead of CV_Assert:
- CV_Assert with && 'message' doesn't display the message correctly
- CV_Error properly reports cv::Error::StsNullPtr with clear message
2. Fixed test kernels to properly initialize state using std::make_shared:
- GOCVStInvalidResize: Initialize state in setup()
- GOCVCountStateSetups: Initialize state before incrementing counter
- Used std::make_shared<int>() for modern C++ best practice
Impact:
- Prevents crashes on platforms with strict null pointer checking
- Provides actionable error message for developers
- Fixes StatefulKernel.StateInitOnceInRegularMode and InvalidReallocatingKernel tests
G-API: Make test execution lighter (first attempt) #25060
### Background
G-API tests look running longer than tests for the rest of modules (e.g., 5m), and the analysis show that there's several outliers in G-API test suite which take a lot of time but don't improve the testing quality much:

In this PR I will cut the execution time to something reasonable.
### Contents
- Marked some outliers as `verylong`:
- OneVPL barrier test - pure brute force
- Stateful test in stream - in fact BG Sub accuracy test clone
- Restructured parameters instantiation in Streaming tests to generate less test configurations (54 -> 36)
### Pull Request Readiness Checklist
See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request
- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake
Fixed handling of new stream, especially for stateful OCV kernels
* Fixed handling of new stream, especially for stateful OCV kernels
* Removed duplication from StateInitOnce tests
* Addressed review comments for PR #21731
- Fixed explanation comments
- Expanded test for stateful OCV kernels in Regular mode
* Addressed review comments for PR #21731
- Moved notification about new stream to the constructor
- Added test on state reset for Regular mode
* Addresed review comments
* Addressed review comments
Co-authored-by: Ruslan Garnov <ruslan.garnov@intel.com>
G-API: Removing G-API test code that is a reflection of ts module
* gapi: don't hijack testing infrastructure
* Removed initDataPath functionality (ts module exists)
* Removed false for ocv_extra data from findDataFile
Co-authored-by: Alexander Alekhin <alexander.a.alekhin@gmail.com>