tar: use diffutils for ptest instead of busybox

tar: use diffutils for ptest instead of busybox

A testcase (sparse03) sometimes times out on the AB, in qemu (without kvm):
the test generates an 8GB sparse file, tars it, untars it, and then
it compares the two versions with cmp.

This process, going through 16GB of data (using one thread, with cmp) takes some
time anyway, but when there is extra load on the host machine, and qemu
can't use its core exclusively, then it can take more than 5 minutes easily
(which is the default ptest timeout).

However the full version of cmp from diffutils seems to be more efficient than
the busybox version:

When using busybox on my idle machine (w/ qemuriscv64) the test case execution
takes 150s, and it almost always times out when there is extra load.

Using diffutils, my idle machine executes the same testcase in 55s, and it
never times out even if there is high load on the host system (execution
always stayed under 3 minutes).

Due to this switch to diffutils when running ptest.

Fixes [YOCTO 15884]

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>