Tidy up DecimalFormatTest

The intent is to clean up the code to make it more obvious
where things are failing and why. The names of the tests now
better reflect their purpose and the code is reformatted.

Some tests have been merged where they overlapped and some
have been split.

(cherry picked from commit 8f4123761cebf7a925b1df887282014e420ac14e)

Change-Id: Ia0e99d3620c30933f9a6ed1143b915dd843e521e
1 file changed