test-sysusers: fix how paths are calculated

test-sysusers: fix how paths are calculated

We were looking at ${f%.*}, i.e. the $f with any suffix starting with a dot removed.
This worked fine for paths like /some/path/test-11.input. It also worked
for paths like /some/path/inline (there were no dots, so we got $f back unscathed).
But in the ubuntu CI the package is built in a temporary directory like
/tmp/autopkgtest-lxc.nnnfqb26/downtmp/build.UfW/ (yes, it has a dot, even two.).
That still worked for the first case, but in the second case we truncated things
after the first dot, and we would try to get
/tmp/autopkgtest-lxc.nnnfqb26/downtmp/build and try to load
/tmp/autopkgtest-lxc.nnnfqb26/downtmp/build.expected-password, which obviously
didn't work as expected. To avoid this issue, do the suffix removal only when
we know that there really is a suffix.

A second minor issue was that we would try to copy $1.expected-*, and sometimes
$1 would be given, and sometimes not. Effectively we were relying on there
not being any files matching .expected-*. There weren't any such files, but let's
avoid this ugliness and always pass $1.