Refactor test parseISO.js to increase test coverage

[dilijev]

test\Date\parseISO.js contains a lot of test cases (and generates a lot of output) and thus is tagged Slow. Unfortunately it is also excluded from Jenkins and xplat for various reasons (timezone and negative years):

  <test>
    <default>
      <files>parseISO.js</files>
      <baseline>parseISO.baseline</baseline>
      <!-- test is timezone-sensitive; remove exclude_jenkins after fix (Microsoft/ChakraCore#319) -->
      <!-- OSX doesn't provide expected DST for minus years -->
      <tags>exclude_jenkins,exclude_xplat,Slow</tags>
    </default>
  </test>

Thus, this test is only run in Nightlies and perhaps doesn't get the attention it deserves. This test could be split into 3 tests: timezone-sensitive, negative years, and everything else. Perhaps at that time it will not be meaningfully slow and we can remove the Slow tag, enabling these tests in more test configurations.

As for the Slow tag:

This test takes 4 seconds to run on my machine, which probably means it will run for a concerning amount of time on CI VMs. Hopefully, when the test is split into parts it will be faster.