Fix ISO8601UtilsTest failing on systems with UTC+X

[Marcono1234]

Previously ISO8601UtilsTest.testDateFormatString() would fail on systems where the time zone is UTC+X because getTime() returned "2018-06-24" for them.

Additionally the tests which previously changed the system locale and time zone have been rewritten to create a UTC calendar instead. Setting locale seems to not be necessary because ISO8601Utils.parse(...) does not do that either.

[LorenzNickel]

Hey, I had the same problem while running the test and also created a PR for it (#1665), where I use another approach to the same problem

[Marcono1234]

Thanks for pointing this out, I did not search before creating the pull request, which I should probably have done.

I am biased since I created this pull request, but I prefer this approach because

• It also sets Locale.US, so it behaves the same as ISO8601Utils.format, not sure if otherwise the test could fail for certain Locales
• It removes changing the system time zone and locale since that is error-prone if a test forgets to reset them and also prevent concurrent use

[eamonnmcmanus]

Thanks! I agree that the approach here seems reasonable, even if it is a bit more complicated than the simplest possible fix.