Create autopkgtest

[Nathan Teodosio]

Ran successfully with autopkgtest -B . -- schroot noble-amd64.

[Paride Legovini (paride)]

There are issues with the proposed diff.

1. the d/changelog entry is not descriptive of what the added test does. Please add something like (you'll know better):

 * d/tests: add basic connection test (LP: #....)

2. the debdiff adds d/t/a.sh which is unused.

3. d/t/connect: better not specify shell options in the shebang, as that get lost if the script is invoked with `sh /path/to/script`. Use `set`.

4. d/t/connect: isNaturalNumber() returns 0 on the empty string, but we don't want that.

5. d/t/connect: this logic:

if isNaturalNumber "$port"; then
    exit 1
fi

seems backwards to me: don't we want $port to be a natural number? FWIW I think it's enough to check that it's a nonempty string ([ -n "$port" ]).

6. xrdp.service has WantedBy=multi-user.target. Why do we need to manually `sudo systemctl start xrdp`?

7. Can we make the `timeout 4s` longer? In general, can you add a comment on what we are testing more precisely?