Adding unit test to reproduce the data leakage issue

[kieranbenton]

Reproduces #216

Beware - ive found that this test (and others?) flap, no doubt because the unit tests are running by default with some parallelism.

If you run this unit test in isolation it fails 100% of the time.

[SebastianStehle]

Tyvm. Please check the formatting of other tests and see if you rely need that much mjml to reproduce it.

[kieranbenton]

Tyvm. Please check the formatting of other tests and see if you rely need that much mjml to reproduce it.

Not sure what you mean about formatting of other tests? I found Should_produce_deterministic_results which seemed appropriate and have added a test alongside that?

[SebastianStehle]

But the mjml formatting is super hard to read. I actually think the problek is that the second example contains the mjml twice, which caused a reuse of some buffers. But it too me ages to see that because the mjml is just one long line. The errors in the second block probably do not matter.