derivations: Adjust tests for Stow environments.

Fixes <https://bugs.gnu.org/30250>.
Reported by Jorge <jorge+list@disroot.org>.

* tests/derivations.scm ("add-to-store, flat")
("add-to-store, recursive"): Call 'readlink*'.

tests/derivations.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -96,7 +96,10 @@
 (test-skip (if %store 0 12))
 
 (test-assert "add-to-store, flat"
-  (let* ((file (search-path %load-path "language/tree-il/spec.scm"))
+  ;; Use 'readlink*' in case spec.scm is a symlink, as is the case when Guile
+  ;; was installed with Stow.
+  (let* ((file (readlink*
+                (search-path %load-path "language/tree-il/spec.scm")))
          (drv  (add-to-store %store "flat-test" #f "sha256" file)))
     (and (eq? 'regular (stat:type (stat drv)))
          (valid-path? %store drv)
@@ -104,7 +107,9 @@
                  (call-with-input-file drv get-bytevector-all)))))
 
 (test-assert "add-to-store, recursive"
-  (let* ((dir (dirname (search-path %load-path "language/tree-il/spec.scm")))
+  (let* ((dir (dirname
+               (readlink* (search-path %load-path
+                                       "language/tree-il/spec.scm"))))
          (drv (add-to-store %store "dir-tree-test" #t "sha256" dir)))
     (and (eq? 'directory (stat:type (stat drv)))
          (valid-path? %store drv)