If one has an object.builder file in the current directory and runs
test_ringbuilder, it will fail with an irritating error. That's because
test_use_ringfile_as_builderfile doesn't use self.tmpfile, but
object.builder - and that one might exist in the local directory.
This patch changes this, using self.tmpfile as argument name.
Closes-Bug: 1590356
Change-Id: I4b3287a36e8a5e469eb037128427dc7867910e53
f6b0b75a25