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
d8173f21).

* 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'.