L::C::Package: Clarify that dirs needs trailing slashes

Signed-off-by: Niels Thykier <niels@thykier.net>
c/testsuite{,.desc}: New check to test for autopkgtest mistakes

[nthykier: add test cases for the check]

Signed-off-by: Niels Thykier <niels@thykier.net>