Functional test for derivation "advanced attrs"

This tests the Nix language side of things.

We are purposely skipping most of `common.sh` because it is overkill for
this test: we don't want to have an "overfit" test environment.

Co-Authored-By: John Ericson <John.Ericson@Obsidian.Systems>

tests/functional/common/paths.sh

@@ -0,0 +1,20 @@
+# shellcheck shell=bash
+
+commonDir="$(readlink -f "$(dirname "${BASH_SOURCE[0]-$0}")")"
+
+# Since this is a generated file
+# shellcheck disable=SC1091
+source "$commonDir/subst-vars.sh"
+# Make sure shellcheck knows this will be defined by the above generated snippet
+: "${bindir?}"
+
+export PATH="$bindir:$PATH"
+
+if [[ -n "${NIX_CLIENT_PACKAGE:-}" ]]; then
+  export PATH="$NIX_CLIENT_PACKAGE/bin":$PATH
+fi
+
+DAEMON_PATH="$PATH"
+if [[ -n "${NIX_DAEMON_PACKAGE:-}" ]]; then
+  DAEMON_PATH="${NIX_DAEMON_PACKAGE}/bin:$DAEMON_PATH"
+fi