Add a helper test method for checking deserialization
Add a helper test method for checking deserialization

It tests if a deserialization method fails properly when an invalid
input is given.

Signed-off-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>
Reviewed-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>

leshchevds/ganeti,leshchevds/ganeti,mbakke/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,dimara/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,ganeti/ganeti,ganeti/ganeti,dimara/ganeti,apyrgio/ganeti,andir/ganeti,grnet/snf-ganeti,leshchevds/ganeti,yiannist/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,apyrgio/ganeti,bitemyapp/ganeti,onponomarev/ganeti,ganeti/ganeti,andir/ganeti,onponomarev/ganeti,mbakke/ganeti,andir/ganeti