Fix a flaky test for Darwin converters (#288)

Fixes #287

Explicitly format the date in the test with the UTC timezone

Author: MarkCMann

core/darwin/test/ConvertersTest.kt

@@ -79,6 +79,7 @@ class ConvertersTest {
         components.timeZone = utc
         val nsDate = gregorian.dateFromComponents(components)!!
         val formatter = NSDateFormatter()
+        formatter.timeZone = utc
         formatter.dateFormat = "yyyy-MM-dd"
         assertEquals("2019-02-04", formatter.stringFromDate(nsDate))
     }