tests/migration-test: Fix "true" vs true

tests/migration-test: Fix "true" vs true

Accidental use of "true" as a boolean; spotted by coverity
and Peter.

Fixes: b99784ef6c3
Fixes: d795f47466e
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Coverity (CID 1432373, 1432292, 1432288)
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210504100545.112213-1-dgilbert@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>