Layer test

[NALLEIN]

Pull Request Readiness Checklist

Fix #329
The test method is as follows
Set test data path

git clone https://github.com/opencv/opencv_extra.git
export OPENCV_TEST_DATA_PATH=${PATH_TO}/opencv_extra/testdata

Test ops

git clone https://github.com/NALLEIN/opencv.git
cd opencv/
git checkout -b layerTest origin/layerTest
mkdir build
cd build
cmake -D CMAKE_BUILD_TYPE=Release -DWITH_WEBGPU=ON -D CMAKE_INSTALL_PREFIX=/usr/local ..
make -j8
$(PATH_TO_OPENCV)/build/bin/opencv_test_dnn --gtest_filter=*layers*

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under OpenCV (BSD) License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[huningxin]

@NALLEIN , after PR #338 merged, please clean this PR and rebase to gsoc_2020_gpu branch.

@wzw-intel , please help review the ops implementation.

[NALLEIN]

Now the WGPU backend could pass all the test case about layers using following instructions:
Set test data path

git clone https://github.com/opencv/opencv_extra.git
export OPENCV_TEST_DATA_PATH=${PATH_TO}/opencv_extra/testdata

Test ops

git clone https://github.com/NALLEIN/opencv.git
cd opencv/
git checkout -b layerTest origin/layerTest
mkdir build
cd build
cmake -D CMAKE_BUILD_TYPE=Release -DWITH_WEBGPU=ON -D CMAKE_INSTALL_PREFIX=/usr/local ..
make -j8
$(PATH_TO_OPENCV)/build/bin/opencv_test_dnn --gtest_filter=*layers*

@huningxin @wzw-intel PTAL.

[NALLEIN]

Done in upstream PR.