test convert Makefile to CMake (#2495)

* Convert tests/Makefile to a cmake project file.

This change switches the building of LVGL tests to use cmake
which is more portable than make. Additionally, whenever
cmake can be used, instead of the Python script (`main.py`),
the former is preferred.

The interface to `main.py` is unchanged, and tests are built
and executated the same as before.

This closes https://github.com/lvgl/lvgl/issues/2474.

* Installing `gcovr` in GitHub workflow.

* Documented steps to install libpng-dev.

* Added missing stdout flush when running tests.

* Grammar tweak in README.

Co-authored-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>

.gitignore

@@ -19,3 +19,6 @@ out_html
 __pycache__
 /emscripten_builder
 test_screenshot_error.h
+build/
+tests/build_*/
+tests/report/