Annoying double-slash in `TestDirectory` output

[fingolfin]

Recently messages printed by TestDirectory started to look like this:

/home/runner/gap/pkg/hap-1.47//tst/...

Notice the double slash. Not terrible bad, but annoying.

I suspect (but did not check, and thus might be totally wrong) that PR #5178 by @zickgraf could have started this.

If right, then one way to changes this would be to adjust things to be kind of the opposite: instead of ensuring there is always a trailing slash, we could ensure there is never a trailing slash; I think both are fine as long as we are consistent.

Another fix would be to adjust the TestDirectory code specifically to avoid the double slash. But I wonder if there are other places that might be similarly affected...

[zickgraf]

If right, then one way to changes this would be to adjust things to be kind of the opposite: instead of ensuring there is always a trailing slash, we could ensure there is never a trailing slash; I think both are fine as long as we are consistent.

I think having the trailing slash is a good thing because it ensures compatibility with Directory, which always adds a trailing slash. So if other places are affected, I think they should simply be changed to use Directory and Filename, which should be more stable and consistent than handling file paths as strings.