Fixes a regression introduced in
af57d1bf6c46f47d82dbc234dde1e16fa8634e9d whereby tests would end up
building the world.
* guix/gexp.scm (mixed-text-file): Add #:guile parameter and honor it.
* tests/gexp.scm ("mixed-text-file"): Pass #:guile to 'mixed-text-file'.
("file-union"): Pass #:guile to 'file-union'.
("lower-object, computed-file"): Pass #:guile to 'computed-file'.
("lower-object, computed-file + grafts"): Likewise.
* tests/packages.scm ("origin->derivation, single file with snippet"):
Likewise.
* tests/profiles.scm ("profile-derivation, ordering & collisions"):
Likewise.
* guix/tests.scm (test-file): Likewise.
