Update checker test should avoid touching git track files #504

From Hung-Chih:

The "lastversioncheck” timestamp entry in the file “packages/cli/test/fixtures/lastversioncheck/package.json” can change from time to time after a some Rush command.

This is indeed a weird thing where you have to be careful not to check this in after you run a test. We should fix it so it doesn't touch the file in the test.