Fixed OpenCV issue #18878

original commit: b82700ae

- avoid randomized output due to unpredictable dict/set order

The character literal overload is more efficient. More info at:

http://clang.llvm.org/extra/clang-tidy/checks/performance-faster-string-find.html

original commit: 4699d2ba

 If provided `PyObject` can't be converted to string `TypeError` is
 reported instead of `SytemError` without any message.

Applied '@ref' linking for 3.4 Calib3D parameters and added enum cv::fisheye::CALIB_ZERO_DISPARITY == cv::CALIB_ZERO_DISPARITY == 0x400 == 1 << 10.
Fisheye test has been updated to use new enum cv::fisheye::CALIB_ZERO_DISPARITY and included CV_StaticAssert(...) to ensure cv::CALIB_ZERO_DISPARITY == cv::fisheye::CALIB_ZERO_DISPARITY.

- dst is not initialized, may include NaN values
- 0*NaN produces NaN

- configure through OPENCV_TEST_VIDEOIO_BACKEND_REQUIRE_FFMPEG environment variable

- support modern Emscripten build process
- replaced Docker image
- replaced Emscripten's web URLs