Disable tests in Travis configuration.

As cppunit is not available, building tests fails.

Also disable optimizations in an attempt to make the build faster as we risk
being killed because it takes so long.

Finally, test compilation of the minimal sample using the installed library
for completeness.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74599 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775