Run make obj before running make test-includes

Before this change, make test-includes (run as part of buildworld)
would place test files in the current directory, which would clutter up
git clones. Run make obj beforehand to ensure that the files are put
in ${.OBJDIR} instead of ${.CURDIR}. This helps cut down on the
noise significantly when running commands like git status.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55499