λ1012 code fix: Fix-All needs a second pass when three or more fixes add the same using #1020

Same defect as #1009, in a rule that shipped in Funcky.Analyzers 2.0.0: OrNone/PreferOrNoneCodeFix.cs (λ1012, prefer …OrNone over …OrDefault with a null check) inserts using Funcky.Extensions; through its own path with the default BatchFixer. Three or more diagnostics in one document that all need the using collide in the batch fixer, so some invocations are only rewritten by a second Fix-All pass.

Fix: derive from the ExtensionMemberCodeFix base introduced by PR #1013 (document-based Fix-All provider, rewrites all flagged invocations in one pass, adds the using once), and extend the λ1012 tests to three or more diagnostics per document. Found during the review of #1013.