selftests: change install command to rsync

The command of install could not handle the special files in exec
testcases, change the default rule to rsync to fix this.

The installation is unchanged after this commit.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>