lightningd: test finding plugins path

[Lagrang3]

Description

Adds unit test to function relative that is used to compute the path to locate the builtin plugins and subdaemons.

Related Issues

It follows after #7618

[rustyrussell]

We don't use path_simplify because it requires the path exist. Pretty sure I tried to make this change before and discovered I had broken things!

In fact, see the last commit message:

    And we can't use path_rel, because that requires that the path
    exist (thanks @Lagrang3!).

You were the one who pointed this out?

[Lagrang3]

I think I said one couldn't use path_rel from ccan because it was relying on a kernel function realpath that in turns
requires the path to exist.
path_simplify on the other hand only calls lstat to do something (not sure exactly what) with symlinks. But it seems fine
if the path does not exist.
I'll double check this. But the tests seem to work.

UPD. path_simplify would not work correctly if the path does not exist, for example
path_simplify("/fakedir/..") yields /fakedir instead of /
and path_simplify("/fakedir/../..") triggers an assertion.