Run compilation failures tests on Travis

"failtest" target is not built by default and it's not trivial to change
this, as it needs to be built before anything else (and not in parallel
with it, as otherwise object files created while building the tests
could be removed), so just build it manually before building the tests
themselves in Travis CI builds (or at least those of them using
configure and make).

build/tools/travis-ci.sh

@@ -62,6 +62,7 @@ case $wxTOOLSET in
         echo -en 'travis_fold:end:script.build\\r'
 
         echo 'Building tests...' && echo -en 'travis_fold:start:script.tests\\r'
+        make -C tests $wxJOBS failtest
         make -C tests $wxJOBS
         echo -en 'travis_fold:end:script.tests\\r'