test: bootm: Ensure tests can be run twice

test: bootm: Ensure tests can be run twice

Some of the bootm tests rely on state that is assumed to be
correct but is changed by the tests. This means that running
'ut bootm' twice will result in failures on the second run
as the state left by the first run is not what the tests
expect.
Fix this by ensuring the state is as expected by explicitly
setting that state.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>