Implement autopkgtest.

Set header in debian/control.

debian/tests/pkg-perl/SKIP: skip use.t.
`perl -w -MFoo::Bar -e1' doesn't find the modules in the private
directories. Actually, without debian/tests/pkg-perl/use-name it wouldn't
even try but this was added in case META.* appears in a later release.

smoke tests:
debian/tests/pkg-perl/smoke-env: set PERL5LIB so the tests find the modules.
debian/tests/pkg-perl/smoke-files: we need more than t/.
debian/tests/pkg-perl/smoke-skip: skip tests which still fail.
This needs further investigation.
debian/patches/autopkgtest.patch: adjust path to scripts when run under
autopkgtest.

syntax.t: add empty debian/tests/pkg-perl/syntax-skip to that all modules
are tested although there are Suggests in debian/control.