test: bootm: Ensure GD_FLG_SILENT is reset

test: bootm: Ensure GD_FLG_SILENT is reset

Some bootm tests expect that GD_FLG_SILENT is reset in order
to work as expected. This is the state if the test is run with
'pytest --verbose' but not if run from, say, the sandbox command
line.
So reset the flag for those tests that rely on it being reset. This
has to be done in each test as the test infrastructure will set it
again before every test when not invoked with 'pytest --verbose'.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>