Fix namespace font locking

[Bost]

• The commits are consistent with our [contribution guidelines][1].
• You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!

I added only some visual tests in the test.clj and also their ert variants. M-x spacemacs/ert-run-tests-buffer invoked on test/clojure-mode-font-lock-test.el produces no new errors:

Selector: (satisfies (lambda (test) (eq cbuf (spacemacs//find-ert-test-buffer test))))
Passed:  31
Failed:  4 (4 unexpected)
Skipped: 0
Total:   35/35

Started at:   2017-11-06 15:53:08+0100
Finished.
Finished at:  2017-11-06 15:53:11+0100

F.F.......F.............F..........

F clojure-mode-syntax-table/characters
    (ert-test-failed
     ((should
       (eq
        (clojure-test-face-at 1 2 "\\1")
        'clojure-character-face))
      :form
      (eq various-faces clojure-character-face)
      :value nil))

F clojure-mode-syntax-table/comment-macros
    (ert-test-failed
     ((should
       (equal
        (clojure-test-face-at 5 41 "#_ 
;; some crap
 (lala 0101
 lao

 0 0i)")
        'font-lock-comment-face))
      :form
      (equal various-faces font-lock-comment-face)
      :value nil :explanation
      (different-atoms various-faces font-lock-comment-face)))

F clojure-mode-syntax-table/fn
    (ert-test-failed
     ((should
       (eq
        (clojure-test-face-at 2 3)
        'font-lock-keyword-face))
      :form
      (eq t font-lock-keyword-face)
      :value nil))

F clojure-mode-syntax-table/ns-macro
    (ert-test-failed
     ((should
       (eq
        (clojure-test-face-at 1 10 "[ns name]")
        nil))
      :form
      (eq various-faces nil)
      :value nil))

• You've run M-x checkdoc and fixed any warnings in the code you've written.
• You've updated the changelog (if adding/changing user-visible functionality).
• You've updated the readme (if adding/changing user-visible functionality).

I fixed a bug not previously reported. No readme change is needed.
I'll try to resolve the supposed redundancy between lines 858 ... 872 in a future PR.

[bbatsov]

I'll close this PR, as I like better your approach in #462. Let's just focus on it.