Merge branch 'jk/filter-process-fix'

Test portability improvements and cleanups for t0021.

* jk/filter-process-fix:
t0021: fix filehandle usage on older perl
t0021: use $PERL_PATH for rot13-filter.pl
t0021: put $TEST_ROOT in $PATH
t0021: use write_script to create rot13 shell script

diff --cc t/t0021-conversion.sh
Simple merge