clan/checks: move into lib function; add tests

lib/clan/checkConfig.nix

@@ -0,0 +1,19 @@
+{ lib, ... }:
+/**
+  Function to assert clan configuration checks.
+
+  Arguments:
+
+  - 'checks' attribute of clan configuration
+  - Any: the returned configuration (can be anything, is just passed through)
+*/
+checks:
+lib.deepSeq (
+  lib.mapAttrs (
+    id: check:
+    if check.ignore || check.assertion then
+      null
+    else
+      throw "clan.checks.${id} failed with message\n${check.message}"
+  ) checks
+)