t3901: move supporting files into t/t3901/

t3901: move supporting files into t/t3901/

The current convention is to either generate files on the fly in tests,
or to use supporting files taken from a t/tNNNN/ directory (where NNNN
matches the test's number, or the number of the test from which we
borrow supporting files).

The test t3901-i18n-patch.sh was obviously introduced before that
convention was in full swing, hence its supporting files still lived in
t/t3901-8859-1.txt and t/t3901-utf8.txt, respectively.

Let's adjust to the current convention.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>