tests: Disable grafting by default for most tests.

This allows tests to run as expected even in the presence of
replacements among the bootstrap packages, such as Perl (commit
d8173f21f7).

* tests/cpan.scm: Add (%graft? #f).
* tests/derivations.scm: Likewise.
* tests/graph.scm: Likewise.
* tests/monads.scm: Likewise.
* tests/profiles.scm: Likewise.
* tests/gexp.scm: Likewise.
("gexp->derivation vs. grafts"): Explicitly reenable grafting before,
and disable it after, using 'set-grafting'.

tests/graph.scm

@@ -24,6 +24,7 @@
   #:use-module (guix derivations)
   #:use-module (guix store)
   #:use-module (guix monads)
+  #:use-module (guix grafts)
   #:use-module (guix build-system gnu)
   #:use-module (guix build-system trivial)
   #:use-module (guix gexp)
@@ -41,6 +42,9 @@
 (define %store
   (open-connection-for-tests))
 
+;; Globally disable grafts because they can trigger early builds.
+(%graft? #f)
+
 (define (make-recording-backend)
   "Return a <graph-backend> and a thunk that returns the recorded nodes and
 edges."