Now that vtc.typesize() is a bit more reliable, it's easier to deal with
architecture differences for the sizeof struct vrt_blob. For a much more
reliable failure check, it now looks at logs.

There may also have been a time when depending on the error we trigger
we could fail with a 500 or a 503, this is no longer the case.

For the nitty-gritty of the new test case, there are some comments to
hopefully help (at least my future self) decipher this beast.

This new test case remains stable even with the changes from #3123.