fix(test): resolve busted migration test isolation issues

Problem: Two issues introduced during the plenary-to-busted migration
(6be0148). First, altbuf_spec waited for two BufEnter events but
oil:// → file resolution only fires one async BufEnter (the synchronous
one from vim.cmd.edit fires before wait_for_autocmd is registered).
Second, reset_editor kept the first window which could be a preview
window with stale oil_preview/oil_source_win state, causing
close_preview_window_if_not_in_oil to close the wrong window in
subsequent tests.

Solution: Wait for a single BufEnter in the altbuf test. Replace the
window-keeping logic in reset_editor with vim.cmd.new() +
vim.cmd.only() to guarantee a fresh window with no inherited state.
This also fixes the preview_spec.lua:30 timeout which had the same
root cause. 114/114 tests pass.

spec/altbuf_spec.lua

@@ -22,7 +22,6 @@ describe('Alternate buffer', function()
     local readme = fs.join(vim.fn.getcwd(), 'README.md')
     vim.cmd.edit({ args = { 'oil://' .. fs.os_to_posix_path(readme) } })
     test_util.wait_for_autocmd('BufEnter')
-    test_util.wait_for_autocmd('BufEnter')
     assert.equals(readme, vim.api.nvim_buf_get_name(0))
     assert.equals('foo', vim.fn.expand('#'))
   end)