tests: reduce unnecessary rebuilds of several tests

Some test were referring to the whole source code via ${self} which amde them rebuild on every single commit.

This is not mitigated by introduceing `self.filter { include = [...]; }` allowin to a content addressed subset of the source code in tests.

clanModules/flake-module.nix

@@ -1,6 +1,13 @@
-{ ... }:
+{ lib, ... }:
+let
+  inherit (lib)
+    filterAttrs
+    pathExists
+    ;
+in
 {
-  flake.clanModules = {
+  # only import available files, as this allows to filter the files for tests.
+  flake.clanModules = filterAttrs (_name: pathExists) {
     admin = ./admin;
     borgbackup = ./borgbackup;
     borgbackup-static = ./borgbackup-static;