t/: Fix the test_install_syntax check

Due to a silly copy & paste mistake, two checks in this case were
testing dh-exec-subst instead of dh-exec-install.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
dh-exec-install: Make it work on unqualified install files too

dh-exec-install should work on all files dh_install itself does, and
that includes debian/install, without the package name too.

Closes: #698017
Reported-by: Axel Beckert <abe@debian.org>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
debian/changelog: Updated to include latest changes

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>