Tidy up of the Base64Test

As they were written the tests would be hard to debug:
non-ASCII bytes in decoder output were being elided to the
'�' char in Strings, which would make it hard to work out
what bytes the decoder was actually returning. Checks
have been added to make sure tests that use Strings
for compactness stick within the ASCII range. Other
tests just use int[]. Some redundant tests have been
removed.

Change-Id: Iaa8997840944e74d85ed24ded028dd841af7b2ca
1 file changed