🤖 Add average test runner run time to config.json (#367)

This commit adds a new `test_runner.average_run_time` key to the `config.json` file. The purpose of this field is allow the website to show a progress bar while the test runner runs. The average run time is defined in seconds with one digit of precision. Its value has been populated by timing the execution time of running the test runner in Docker on one of the track's practice exercise. Clearly, the execution time will differ between exercises and between solutions, so this should be seen as a general indicator.

config.json

@@ -10,6 +10,9 @@
     "ace_editor_language": "elm",
     "highlightjs_language": "elm"
   },
+  "test_runner": {
+    "average_run_time": 2.0
+  },
   "status": {
     "concept_exercises": false,
     "test_runner": true,