build-and-provide-package: Look only for the "real" debian/control file

Otherwise this breaks in a situation like that:

| % tar atf autopkgtest_2.3.5~pgdg70+1.tar.gz | grep control
| -rw-r--r-- 0/0             297 2013-09-25 08:51 source/tests/testpkg/debian/control
| -rw-r--r-- 0/0             151 2013-09-25 08:51 source/debian/tests/control
| -rw-r--r-- 0/0            1982 2013-09-25 08:51 source/debian/control

Thanks: Christoph Berg <christoph.berg@credativ.de> for the bugreport and fix