Build tests by default

The tests are now built by default. This should make it easier for users
to verify that they have a working MaxScale.

Also made the building of test_parse_kill conditional like the rest of the
tests.

server/modules/protocol/MySQL/CMakeLists.txt

@@ -5,4 +5,7 @@ install_module(mysqlcommon core)
 
 add_subdirectory(mariadbbackend)
 add_subdirectory(mariadbclient)
-add_subdirectory(test)
+
+if (BUILD_TESTS)
+  add_subdirectory(test)
+endif()