testfixture: PRAGMA encoding ignored, text is fixed at UTF-8 (UTF-16 databases unsupported)

`test/known_testfixture_exception_inventory.txt` marks 6 inherited SQLite
testfixture suites `unsupported` because DoltLite's text encoding is fixed at
UTF-8. `PRAGMA encoding` is ignored and UTF-16 databases are not supported, so
the encoding-conversion and misaligned-UTF-16 suites cannot pass.

Prolly-tree sort keys are content-addressed and must be byte-identical across
branches for two independently-written identical rows to share an identity, so
a per-database text encoding is not something the storage engine can carry.
Not expected to be fixed. The issue exists so the inventory's `unsupported`
entries point at a live description of the limitation; they previously pointed
at #1164, which is closed.

Affected suites (6): badutf enc enc2 enc3 enc4 utf16align

Open test/known_testfixture_exception_inventory.txt and locate the unsupported entries for badutf, enc, enc2, enc3, enc4, and utf16align. Confirm they describe the UTF-8-only limitation and update references from the closed #1164 to this issue, then review the diff to ensure all six entries remain marked unsupported.

Written by the indexing model from the issue text.