Graph the number of test coverages across time to easily spot the drop of test coverage

FDB code has embedded `TEST()` statement to specify the coverage of a code block (or a code path).

It is important that these `TEST()` statements are fully exercise in simulation so that we are certain those code blocks have been tested.

Sometimes, we may turn off some tests temporarily and forgot to turn those tests back on. This will leave some code untested without notice because simulation does not report error for the lack of coverage.

To make it easier and obvious to spot those kinds of mistakes that may reduce the test coverage, graphing the number of coverages (and the exercise times per coverage type) across time can be useful.

[This is from an offline discussion when Evan is talking about simulation.]

The issue names embedded TEST() statements and simulation but no files, tests, or entry points. Start by locating the TEST() coverage data and how simulation exercises it; done should provide a graph of coverage counts and exercise times across time so coverage drops are visible.

Written by the indexing model from the issue text.