Added workflow for Github Actions to build and test OpenCV on Linux for 4.x
* Added workflow for Github Actions to build and test OpenCV
* Merged a build and tests jobs into one, split tests by steps, renamed job names
CI for TIM-VX backend
* github actions for TIM-VX backend https://github.com/opencv/opencv/pull/21036
* add reference to yuentau/ocv_ubuntu:20.04; remove extra quotes; enable BUILD_TESTS
* rename to timvx_backend_tests.yml
* add image source prefix
* remove if condition for x86_64 simulator