Add some form of regression test to measure performance of load times

[DonJayamanne]

We have a unit test or pre-release (manual) task to ensure load times of the extension have not increased (at least not drastically) in each new version.

Three occasions we've identified slow load times of extension:

• Improve performance of automatic environment detection #372
• Speed up interpreter selection (with cached & or partial lists) #666
• Improve extension load time #929

[MikhailArkhipov]

May be hard on CI due to random times there. However, local-only test may work as local runs are relatively consistent - at least on comparable machines.

[brettcannon]

So we can record the number in the test results, but we don't have to succeed/fail based on it (yes that can lead to slow skew, but we are more worried about hard performance loss).