self: Don't build (guix tests …).

Fixes a regression introduced in
03d76577b9 whereby 'guix-extra.drv' would
pull in (guix tests), which in turn would pull in a large number of (gnu
packages …), which would fail to build due to missing .patch files.

* guix/self.scm (compiled-guix)[*extra-modules*]: Exclude (guix tests …)
from the list of modules.

guix/self.scm

@@ -732,6 +732,7 @@ Info manual."
                  (filter-map (match-lambda
                                (('guix 'scripts _ ..1) #f)
                                (('guix 'man-db) #f)
+                               (('guix 'tests _ ...) #f)
                                (name name))
                              (scheme-modules* source "guix"))
                  (list *core-modules*)