tests: mkfs-endianness test uses prepared test data

tests: mkfs-endianness test uses prepared test data

Commit 7b54f05d6b7124c23bff3bc1b8310231c8a2e131 added iflag=fullblock
option, it works fine with coreutils's dd but macOS's dd doesn't support
iflag option then test failed on macOS[1].

This commit added prepared test data for test to not use dd command to
avoid dd command difference.

tested on raspberry pi3
$ sudo sh -c 'for i in $(seq 1 500); do taskset -c 0 ./ts/cramfs/mkfs-endianness ;  done' | grep FAILED | wc -l
0

tested on macOS
$ sudo sh -c 'for i in $(seq 1 500); do ./ts/cramfs/mkfs-endianness ;  done' | grep FAILED | wc -l
       0

[1]https://travis-ci.org/github/karelzak/util-linux/jobs/723642222#L3907

Signed-off-by: Masami Ichikawa <masami256@gmail.com>