core/state: fix the flaky TestSizeTracker

[jsvisa]

closes #32984

After some debugging, I found the flaky was failed of the mismatch of the baseline accounts, in the success test, it showed 52, in the flaky it showed 51:

--- FAIL: TestSizeTracker (0.09s)
    state_sizer_test.go:201: Account count mismatch: baseline(51) + tracked_changes = 131, but final_measurement = 132
    state_sizer_test.go:204: Account bytes mismatch: expected 6758, got 6798
FAIL

As the write buffer in pathdb.Default's async writing, after SnapshotCompleted(), the buffer maybe not fully flushed, then the baseline iterating maybe missed some data.

But in the final result, as we reopened the triedb, so all the buffer have been flushed into db, the result is matched.

Here we propose to reopen the triedb before the baseline iterating.