tests: When a store item is to be deleted, make it statistically unique.

Fixes <http://bugs.gnu.org/18935>.
Reported by Mark H Weaver <mhw@netris.org>.

Previously, if tests/nar.scm ran concurrently with tests/gexp.scm, the
store item "foo" containing the string "Hello, world!" could be
deleted (via 'delete-paths') from the tests/nar.scm daemon while the
tests/gexp.scm daemon would still consider it live.

* tests/nar.scm ("restore-file-set (missing signature)"):
  Use (random-text) rather than "Hello, world!", to avoid concurrent
  deletion of store item "foo" used in tests/gexp.scm.

tests/nar.scm

@@ -277,7 +277,7 @@
 (test-assert "restore-file-set (missing signature)"
   (let/ec return
     (with-store store
-      (let* ((file  (add-text-to-store store "foo" "Hello, world!"))
+      (let* ((file  (add-text-to-store store "foo" (random-text)))
              (dump  (call-with-bytevector-output-port
                      (cute export-paths store (list file) <>
                            #:sign? #f))))