Tweak libcore.libcore.icu.DateIntervalFormatTest#testEndAtMidnight

This CL slightly reformats the tests and breaks it into
two. This ensures that the wrong flags aren't accidentally
used in the wrong part of the test, and avoids some repetition
through use of a helper BiFunction.

Test: atest CtsLibcoreTestCases:libcore.libcore.icu.DateIntervalFormatTest
Change-Id: I64cf12ea8e4bdb9c1f3e36ad3bfaaa2f316590e6
1 file changed