testfixture: shared-cache mode unsupported (connections do not share a pager cache)

`test/known_testfixture_exception_inventory.txt` marks 11 inherited SQLite
testfixture suites `unsupported` because DoltLite does not implement
shared-cache mode. Two connections to the same file do not share a pager cache;
each opens its own file descriptor onto the chunk store, so
`sqlite_open_file_count` and `sqlite3_shared_cache_report` never match the
shared-cache expectations, and the harness setup itself fails in several files.

This is a consequence of replacing the pager and is 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 #1129, which is
closed.

Affected suites (11): fts3shared shared shared2 shared3 shared6 shared7 shared8
shared9 shared_err sharedlock vtab_shared

Start with test/known_testfixture_exception_inventory.txt and inspect the 11 shared-cache suite entries listed in the issue. Update their limitation reference from the closed #1129 to this live issue, keeping them marked unsupported; done means all affected entries point to the current description.

Written by the indexing model from the issue text.