Add gcontext tests

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(Reopening. I've applied the changes to the current master.)

Uncovered problems:

• The setter for gcontext-cache-p doesn't seem to be defined. The manual claims it exists.

  ; caught STYLE-WARNING:
  ;   undefined function: (SETF XLIB:GCONTEXT-CACHE-P)

• Trying to get the gcontext-clip-mask causes an obscure error:

  Failure 2: UNEXPECTED-ERROR when running XLIB-TEST-DISPLAYS::GRAPHICS-CONTEXTS
  The value
    102760450
  is not of type
    (INTEGER 0 0)
  when binding #:G2

• gcontext-line-style is supposed to accept one of these values: :solid :double-dash :on-off-dash according to the manual. In reality, the software seems to assert that it's of type (member :solid :double-dash :dash). Is this to be fixed in the documentation, or in the implementation?
• The function xlib:gcontext-p is undefined. The manual mentions it, though.
• gcontext-stipple may return NIL if no stipple was set (I think). According to the manual, it's supposed to return a pixmap. NIL is not of type pixmap. Fix in the doc?

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Also, @dkochmanski: I'm guessing the comments in the closed PR were meant for some other PR?