public void clearConfigValueMappers() {
        STATIC_VALUE_MAPPERS.clear();
        this.valueMappers.clear();
    }
Use this with care as it will set a mapper that is held statically on the class so will
impact other tests running in the same JVM.
Needed for when you want to set config values that are used in eager singleton construction.
stroom-app/src/test/java/stroom/test/ConfigMapperSpy.java