Put functional tests in tests/functional

[Ericson2314]

Motivation

I think it is bad for these reasons when tests/ contains a mix of functional and integration tests

• Concepts is harder to understand, the documentation makes a good unit vs functional vs integration distinction, but when the integration tests are just two subdirs within tests/ this is not clear.

• Source filtering in the flake.nix is more complex. We need to filter out some of the dirs from tests/, rather than simply pick the dirs we want and take all of them. This is a good sign the structure of what we are trying to do is not matching the structure of the files.

With this change we have a clean:

$ git show 'HEAD:tests'
tree HEAD:tests

functional/
installer/
nixos/

Context

I suppose this is part of #7876

Priorities

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[Ericson2314]

If anyone thinks tests/functional is too long, I am open to other things, include 3 top level dirs (e.g. func-tests nixos-tests installer-tests),

[Ericson2314]

@edolstra was wondering about whether this would induce conflicts. I am happy to report that I made this commit atop this branch, and it cherry-picked it all the way back to 2.3-maintainence with no conflict!

commit 6dfde324fb9dc898a7130932089a3c3553af053e (functional-tests-dir)
Author: John Ericson <John.Ericson@Obsidian.Systems>
Date:   Fri Oct 6 09:06:38 2023 -0400

    asdf

diff --git a/tests/functional/fixed.nix b/tests/functional/fixed.nix
index babe71504..fd5a097f7 100644
--- a/tests/functional/fixed.nix
+++ b/tests/functional/fixed.nix
@@ -34,7 +34,7 @@ rec {
     # But Nix sees that an output with the desired hash already
     # exists, and will refrain from building it.
     (f ./fixed.builder2.sh "flat" "md5" "8ddd8be4b179a529afa5f2ffae4b9858")
-  ];
+  ]; # adsfasdf

   sameAsAdd =
     f ./fixed.builder2.sh "recursive" "sha256" "1ixr6yd3297ciyp9im522dfxpqbkhcw0pylkb2aab915278fqaik";