Force SDL_VIDEODRIVER=dummy for tests

Background: my tests rely on pygame.key.name(K_RCTRL) returning
something meaningful (like "right ctrl").  This function is a thin
wrapper around SDL_GetKeyName(), which looks up a key from a lookup
table.  That lookup table is not static for some reason in SDL 1.2,
and gets initialized by this call chain:

  SDL_KeyboardInit() called by
  SDL_StartEventLoop() called by
  SDL_VideoInit() called by
  SDL_InitSubSystem() called by
  PyGame_Video_AutoInit() called by
  pygame.init() called by
  pygame.tests.test_ui.setUp()

Now SDL_VideoInit() also initializes other things, like the SDL video
subsystem, for which it tries various backends (including X11, aalib and
caca).  This explains why the presence/absence/values of various
environment variables (DISPLAY/TERM) could make or break the tests, or
why it would suddenly and unexpectedly flash and clear the screen for
tox -e coverage.

By forcing a specific SDL video driver (dummy) we remove non-determinism
and hopefully the issue can rest.

.travis.yml

@@ -8,7 +8,7 @@ install:
     - pip install -U pip
     - pip install coverage coveralls -e .
 script:
-    - xvfb-run -a coverage run --source=src/pyspacewar test.py
+    - coverage run --source=src/pyspacewar test.py
 after_success:
     - coveralls
 notifications:

src/pyspacewar/tests/test_ui.py

@@ -219,9 +219,8 @@ def doctest_FrameRateCounter_fps():
 
 def setUp(test=None):
     import pygame
+    os.environ['SDL_VIDEODRIVER'] = 'dummy'
     pygame.init()  # so that pygame.key.name() works
-    # unfortunately, on linux, if $DISPLAY is unset, pygame.init doesn't
-    # complain, but pygame.key.name() returns 'unknown key' for all keys
 
 
 def test_suite():

tox.ini

@@ -7,11 +7,6 @@ envlist =
 install_command = python -m pip install -U {opts} {packages}
 deps =
     pip
-# if DISPLAY is unset on Linux, pygame returns 'unknown key' for all keys
-# which makes tests fail :/
-passenv =
-    DISPLAY
-    TERM
 commands =
     python test.py