[WIP] Add unit tests for GUI Widgets.

[tov101]

• .coveragerc
  • adapted to consider py files from gui folder excepting ui folder.
• coveralls.yml
  • few more libraries needs to be installed on Linux machine in order to get Qt running.
  • display environment needs to be created because job is running in Linux terminal session.
• main.yml
  • few more libraries needs to be installed on Linux machine in order to get Qt running.
  • display environment needs to be created because job is running in Linux terminal session.
  • additional steps are running considering matrix.os value
• wheels.yml
  • cibuildwheel seems to execute the tests by-default, so command was adapted in order to avoid gui tests.
• pyproject.toml`
  • style checks are executed but there differences between py38 and >py38. Freeze to py38 style.
• requirements_exe_build.txt
  • pyqtgraph new release recently which have impact in asammdf. Integration of new release is not in focus of this PR, so freeze lib for now.

Tests:

• file.py -> FileWidget
  • test_Tab_Channels_PushButton_LoadOfflineWindows_DSP
  • test_Tab_Channels_PushButton_LoadOfflineWindows_DSPF
  • test_Tab_Channels_PushButton_LoadOfflineWindows_LAB
  • test_Tab_Channels_PushButton_SaveOfflineWindows
  • test_Tab_Channels_PushButton_SelectAll
  • test_Tab_Channels_PushButton_ClearAll
  • test_Tab_Channels_PushButton_Search