Random testing in Lambda/TestGen

[aqjune-aws]

The random LExpr generator in Strata/DL/Lambda/TestGen.lean fails sometimes. Specifically, it is this test:

    let P : LExpr TrivialParams.mono → Prop := fun t => HasType example_lctx example_ctx t example_ty
    let t ← Gen.runUntil (.some 1) (ArbitrarySizedSuchThat.arbitrarySizedST P 5) 5
    IO.println s!"Generated {t}"

The reason must be identified. #327 makes the test successful by fixing the random seed.