build_image: right size image problems.

Neglected to correct for filesystem size in estimate for verity.

To keep build space optimized and make sure that the existing image
does not confuse the tools, remove the out_file after the test build
image phase.

Test: build
Bug: 111302946
Change-Id: I360775d862d02a721d061bbc43e8b4e2f03bd01e