public <T extends AbstractConfig> void setConfigValueMappers(final Map<Class<T>, UnaryOperator<T>> valueMappers) {
        this.valueMappers.clear();
        this.valueMappers.putAll(valueMappers);
    }
Intercepts any calls to getConfigObject and allows the returned values to be replaced
with different values for testing.
stroom-app/src/test/java/stroom/test/ConfigMapperSpy.java