Print warning when test files take a disproportionate amount of time to declare tests

Based on feedback from @shusson in https://github.com/avajs/ava/issues/2268#issuecomment-676513972 it would be interesting if we could instrument how long it takes between AVA loading the test file to tests being declared.

This time is not taken by AVA itself, but instead by the test and project code. There are likely optimizations possible in the project that is using AVA.

If this takes a disproportionate amount of time of the overall test run (5%? 10%?) we should print a warning after the test run has completed.