Update unit test of InterpreterBlockRule for "a {{{a}}}"

Summary:
This is supposed to render in plain text instead of crashing or giving an error, after D25415:

a {{{a}}}

The related unit test is updated accordingly.

We have unnoticed this, because of the well-known bug T15500.

Ref T15372
See D25415

Test Plan:
This makes sense with the introduced functionality in D25415.

Unit tests are happy again.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15372

Differential Revision: https://we.phorge.it/D25559