test(drop): run past a small search budget instead of writing 50,001 files

[omartelo]

TestResolveRefusesTwinsSplitByTheBudget and TestResolveFindsShallowSiblingBeforeNoise have to run the search past its budget, and the budget was the maxEntries = 50_000 constant, so each wrote 50,001 files. On the Windows runner that took 38s and 37s (the timings table from #802, run 39056236000), most of internal/drop's 111s; on Linux about 1s each.

• maxEntries is now a package var the tests replace, the same idiom as homeDir and sandboxAvailable in the same file. Production still uses 50,000.
• smallBudget(t) sets it to 16 for one test and restores it. Both tests call it; their loops still write budget + 1 entries and their assertions are unchanged, so only the fixture size moves.

Checked: both tests now take 0.00s locally, and breaking the if scan.budget > 0 guard on purpose still fails TestResolveRefusesTwinsSplitByTheBudget with the wrong twin.