public <T extends AbstractConfig> void setConfigValueMapper(final Class<T> clazz,
                                                                final UnaryOperator<T> mutator) {
        this.valueMappers.put(clazz, mutator);
    }
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