config: convert average_run_time to an integer (#291)

There are two reasons for this change:

1. Having the average run time as a float gave an impression of
   exactness, but the actual run time varies significantly for a variety
   of reasons (e.g. how busy it is on the server). The fractional
   component would almost never actually conform to the real situation.

2. jq is often used to work with track config.json files (e.g. to
   add elements to it), and it will remove any trailing .0 fractional
   part from a number, which caused `configlet lint` to fail. Those jq
   scripts would have to work around this by manually adding .0 to it.

config.json

@@ -16,7 +16,7 @@
     "highlightjs_language": "zig"
   },
   "test_runner": {
-    "average_run_time": 5.0
+    "average_run_time": 5
   },
   "files": {
     "solution": [