Unable to cover LtUnlintNonExistingDefect due to hand-written JUnit cases in LtUnlintNonExistingDefectTest

I cannot cover LtUnlintNonExistingDefect through LtByXslTest's pack infrastructure because LtUnlintNonExistingDefectTest still hand-constructs new LtUnlintNonExistingDefect(lints, excluded) in Java for every scenario, the same gap we already closed for LtUnlint in objectionary/lints#1347.

LtUnlintNonExistingDefect is registered in production PkMono under lint: unlint-non-existing-defect, so most of the 12 cases — the ones built on LtAsciiOnly and aliases/alias-too-long fixtures — should convert directly to lint: unlint-non-existing-defect packs with input: EO source. The two cases using the private Boom lint (doesNotRunLintsWithoutUnlints, doesNotInvokeUnreferencedLint) stay hand-written JUnit — Boom only exists to prove an unreferenced lint is never invoked, and it isn't registered in production PkMono, so a pack can't reach it (same reasoning as LtAlways in #1347).

Several of the .eo fixtures this test currently uses (unlint-ascii-only-no-defect.eo, unlint-ascii-only-duplicate.eo, unlint-ascii-only-wrong-line.eo, unlint-ascii-only-out-range.eo, unlint-ascii-only-absent.eo) already violate the current EO grammar — mid-file comments and missing blank lines the parser now rejects — the same class of problem we hit converting LtUnlintTest. These will need rewriting as valid EO, not a literal copy-paste into input:.