Move testfile.tex to the right place
No need to ignore testfile.pdf since it's now taken care of by build.lua
Mention possible improvement.

TeX empties a file-to-be-written when performing \openout, but since
morewrites only performs that primitive operation when the file is
closed, the timing is off.
Remove \AtBeginDocument (targetting compatibility with plain TeX)
Remove redundant commands/comments in testfile.lvt
Hook into shipout properly

Many packages move the shipout primitive, so hooking into shipout is a
bit more subtle than just redefining \shipout.
Add test file