refactor: extract missing-glossary test helper

Four tests across lookup, list, find-buffer-position, and first-call-creates-file shared the same boilerplate. Each let-bound gloss-file to a randomized nonexistent path, wrapped in unwind-protect, reset the cache, and cleaned up file and buffer afterward.

Extracted as gloss-test--with-missing-glossary in testutil-gloss.el, parallel to the existing gloss-test--with-temp-glossary. The four call sites drop from 8-10 lines each to 2-3.

Tested by running the full 32-test suite. All 32 pass in 0.21 seconds.

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