bookmark_sync::incoming::tests::test_apply_folder is unreasonably slow

[mhammond]

That test takes 5s on my box. It does so because it does let children = (1..sql_support::default_max_variable_number() * 2) which is very large. This seems wrong because:

• That really seems to be testing the sqlsupport chunking mechanism, but it's not clear that's the job of that test. If we can ensure the sqlsupport crate has tests for chunking we can probably make this a sane number.
• There doesn't seem to be a good reason for the * 2 - just + 1 would probably test what is necessary and would probably speed it up.

I'm not shaving this yak today, so opening this instead.

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