Fix CharsetTest for SCSU and Shift_JIS.

We went from using approximations to using the real thing, but didn't tell the
tests. I've also added the reverse tests, because that's what we really care
about --- that the byte sequence decodes as the replacement character.

Bug: 10310458

(cherry picked from commit 075bdfb276c6377cc90e568bdfcf6ffb31bed1f6)

Change-Id: I344c5a74c31fea9491bbe43198fde606dac7722c
1 file changed