Travis CI: globally disable C++ unit tests

The tests haven't been run for a couple of months.
The test executable crashes on startup with the error message

Xlib: extension "RANDR" missing on display ":99.0".

After ten attempts, the test wrapper just gives up and returns success.

So, let's not even pretend that we have C++ unit tests running in Travis.
We just don't. Explicitly disabling the tests makes it more clear.

.travis.yml

@@ -36,7 +36,7 @@ before_install:
     - export TARGETS="wesnoth wesnothd campaignd test"
     - export STRICT_COMPILATION=false
     - export EXTRA_FLAGS_RELEASE=""
-    - export WML_TESTS=true CPP_TESTS=true PLAY_TEST=true MP_TEST=true
+    - export WML_TESTS=true CPP_TESTS=false PLAY_TEST=true MP_TEST=true
     - export WML_TEST_TIME=15
 
     - if $NLS; then