Auto-generate Evergreen configuration when adding new 'make check' or csuite tests

In WT-4394 we took the 1st step of splitting 'make check' tests (including csuite tests) manually into separate new Evergreen tasks in the Ubuntu build variant. When new tests, i.e. new test directories, or new files in existing test directories, are added into 'make check' scope later on, we need to build a mechanism to help achieving a smooth development experience. Specifically: 

• a script to auto-generate the evergreen configuration for the newly added tests 
• an approach to detect the addition of new tests during development, consider hooking to dist/s_xxx
• a readme to describe the need of evergreen configuration change when adding new tests, to help with cognitive build