Fix tests and limit internal test set size

Some of the tests depended on a working installation where modules are all
located at the default paths. These tests now explicitly set the module
directory which fixes the immediate problem.

Disabled the starting of services in the service test as this will fail
with real modules. The dummy internal modules aren't build and should be
removed in a later commit. In general, it might be better to do service
level testing outside the internal test suite.

.travis/build_maxscale.sh

@@ -15,6 +15,7 @@ cd build
 cmake .. -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=Debug -DBUILD_TESTS=Y -DBUILD_AVRO=N
 
 make
+make test
 sudo make install
 
 sudo ./postinst