docs(test): add busted migration log

Problem: The plenary-to-busted migration surfaced three test isolation
issues whose root causes and fixes were not documented.

Solution: Add spec/TESTING.md recording the issues, their root causes,
and the applied fixes for future reference. Update CLAUDE.md test
section with correct framework, run command, and test count.

spec/TESTING.md

@@ -0,0 +1,49 @@
+# Test Framework Migration Log
+
+Issues encountered during the plenary-to-busted migration (`6be0148`) and
+how they were resolved.
+
+## Final status
+
+114 successes / 0 failures / 0 errors.
+
+## Issue 1: altbuf_spec.lua:18 — BufEnter wait mismatch (resolved)
+
+The test `sets previous buffer as alternate when editing url file` originally
+waited for two `BufEnter` events via `wait_for_autocmd('BufEnter')` x2.
+
+The oil:// → real file resolution only produces one async BufEnter:
+
+1. `vim.cmd.edit('oil://...')` fires BufEnter synchronously (before any
+   wait_for_autocmd is registered)
+2. `normalize_url` resolves asynchronously via `uv.fs_realpath` + `uv.fs_stat`
+3. `rename_buffer` discovers dest is a real file on disk, schedules
+   `nvim_win_set_buf()` via `vim.schedule()`
+4. `nvim_win_set_buf()` fires one async BufEnter on the real file buffer
+
+The second `wait_for_autocmd('BufEnter')` was never correct — it passed under
+plenary due to coroutine event loop yielding, not because a second BufEnter
+actually fired.
+
+**Fix:** single `wait_for_autocmd('BufEnter')`.
+
+## Issue 2: regression_spec.lua:20 — stale preview window state (resolved)
+
+`reset_editor()` closed all windows except the first, but the surviving window
+could be a preview window with stale `oil_preview`, `oil_source_win`,
+`previewwindow`, etc. from a prior test. Subsequent tests that triggered
+`close_preview_window_if_not_in_oil()` would close the wrong window.
+
+**Fix:** `vim.cmd.new()` + `vim.cmd.only()` creates a fresh window and closes
+all others (including the stale one). No variable cleanup needed — the new
+window has no oil state.
+
+## Issue 3: preview_spec.lua:30 — M.await timed out (resolved)
+
+Originally thought to be a pre-existing issue unrelated to the migration. Root
+cause was identical to issue 2: stale preview window state from a prior test
+caused `close_preview_window_if_not_in_oil()` to close the wrong window during
+`oil.open()`, preventing the callback from completing.
+
+Fixed by the same `vim.cmd.new()` + `vim.cmd.only()` approach in
+`reset_editor()`.