oeqa/runtime/dnf: rewrite test_dnf_installroot_usrmerge

This test doesn't get exercised on the autobuilder and so it was broken:
specifically some of the ln commands silently fail and the chroot isn't
usable.

Rewrite the test case to correctly construct a chroot so the test can
pass.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>