tests: base: Add Guile-Gcrypt & co. to the search path.

Fixes a regression introduced in
ca71942445, whereby (gcrypt hash) would
not be found in the system under test, leading to a failure of the
"/run/current-system is a GC root" test.

* gnu/tests/base.scm (run-basic-test)[guix&co]: New variable.
[test]: Add all of GUIX&CO to %LOAD-PATH.

gnu/tests/base.scm

@@ -42,6 +42,7 @@ (define-module (gnu tests base)
   #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module (srfi srfi-1)
+  #:use-module (ice-9 match)
   #:export (run-basic-test
             %test-basic-os
             %test-halt
@@ -68,6 +69,11 @@ (define special-files
      (fold-services (operating-system-services os)
                     #:target-type special-files-service-type)))
 
+  (define guix&co
+    (match (package-transitive-propagated-inputs guix)
+      (((labels packages) ...)
+       (cons guix packages))))
+
   (define test
     (with-imported-modules '((gnu build marionette)
                              (guix build syscalls))
@@ -345,8 +351,14 @@ (define (entry->list entry)
             'success!
             (marionette-eval '(begin
                                 ;; Make sure the (guix …) modules are found.
-                                (add-to-load-path
-                                 #+(file-append guix "/share/guile/site/2.2"))
+                                (eval-when (expand load eval)
+                                  (set! %load-path
+                                    (append (map (lambda (package)
+                                                   (string-append package
+                                                                  "/share/guile/site/"
+                                                                  (effective-version)))
+                                                 '#$guix&co)
+                                            %load-path)))
 
                                 (use-modules (srfi srfi-34) (guix store))