Fix Makefile to be idempotent

Previously it failed on already existing symlinks.
Add missing dctrl-tools build dependency

Makefile is calling grep-dctrl.
Makefile: Don't call "checkdeps" on "make check"

The test suite doesn't actually need autopkgtest or the libjs packages (which
are for the UI only).
test/test_helper.sh: Don't hardcode "amd64"

This fixes test failure when building on other architectures. E. g. Ubuntu
builds "Architecture: all" packages on i386.