installer: Fix configuration edition during testing.

When the configuration is edited, it looks like there are some leftover
fragments from the input configuration:

Example content of config.scm after edition:

  #:imported-modules
  '((gnu services herd)
    (guix build utils)
    (guix combinators)))

unted".  The unique
  ;; file system identifiers there ("UUIDs") can be obtained
  ;; by running 'blkid' in a terminal.

...

This is strange because call-with-output-file uses the O_TRUNC flag which
resets the file size to zero. Remove the configuration file before writing it
as a work-around.

* gnu/installer/tests.scm (edit-configuration-file): Remove the configuration
file before re-writing it.

gnu/installer/tests.scm

@@ -280,6 +280,10 @@ instrumented for further testing."
        exp)))
 
   (let ((content (call-with-input-file file read-expressions)))
+    ;; XXX: Remove the file before re-writing it, to be sure there are no
+    ;; leftovers.  We shouldn't have to to that as CALL-WITH-OUTPUT-FILE uses
+    ;; the O_TRUNC flag by default.
+    (delete-file file)
     (call-with-output-file file
       (lambda (port)
         (format port "\