gnu: emacs-ert-selector: Minor improvements.

* gnu/packages/emacs.scm (emacs-ert-selector): Surround test names with
beginning of line and end of line regex characters as emacs expects a regex.
Sort the list of tests so that rearranging the excluded tests does not result
in a rebuild.

Change-Id: I7002fa48122d8409f383f85f515a1a6ac9cdd957
Signed-off-by: Liliana Marie Prikler <liliana.prikler@gmail.com>

gnu/packages/emacs.scm

@@ -118,8 +118,9 @@
    (string-join
     (map
      (lambda (test)
-       (string-append "\\\"" test "\\\""))
-     excluded-tests))
+       ;; This is a regex
+       (string-append "\\\"^" test "\\$\\\""))
+     (sort excluded-tests string<?)))
    "))"))
 
 (define %selector