@SuppressWarnings("unused")
        public AssertionsBuilder<I, O> withTestFunction(final Function<TestCase<I, O>, O> testFunction) {
            Objects.requireNonNull(testFunction);
            return new AssertionsBuilder<>(testFunction);
        }
Define the action for the dynamic test lambda, where the action
uses {@link TestCase#getInput()} to produce some output.
Test assertions are added later with {@link AssertionsBuilder#withAssertions(Consumer)}.
stroom-test-common/src/main/java/stroom/test/common/DynamicTestBuilder.java