qemuimagetest/scenario: Move dmesg to end of test run

qemuimagetest/scenario: Move dmesg to end of test run

The dmesg test detects segfaults. This is useful information to have and if one
occurs in one of the earlier tests, this can aid debugging. Move the dmesg test to
the end of the list of tests so we gain the extra debug info in those cases.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>