Both `test_miniquotes_object` and `test_minibrackets_object` rely on
tree-sitter parsing for `MultiBufferSnapshot.bracket_ranges` to find
quote/bracket pairs. The `VimTestContext.set_state` call eventually
triggers async tree-sitter parsing, but `run_until_parked` doesn't
guarantee parsing completion.

We suspect this is what might be causing the flakiness on Windows, as
the syntax might not yet be parsed when the
`VimTestContext.simulate_keystrokes` call is made, so there's no bracket
pairs returned.

This commit adds an explicit await call on `Bufffer.parsing_idle` after
each `VimTestContext.set_state` call, to ensure tree-sitter parsing
completes before simulating keystrokes.

Release Notes:

- N/A