Speed generation of random tests by grouping by zone and invoking date once per zone.
Speed generation of random tests by grouping by zone and invoking date once per zone.

Instead of generating 50000 random tests (random zone and random time),
I now generate 100 random tests per time zone, and invoke date only once
per time zone rather than once for each test.  This speeds generation of
these tests from about 3 minutes and 45 seconds to just under 5 seconds.

dbaron/tz.js,dbaron/tz.js,dbaron/tz.js