copy_buffer_does_not_touch_its_pages intermittent failure

---- tests::copy_buffer_does_not_touch_its_pages stdout ----

thread 'tests::copy_buffer_does_not_touch_its_pages' panicked at src/uu/dd/src/dd.rs:1655:13:
attempt to subtract with overflow
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/std/src/panicking.rs:697:5
   1: core::panicking::panic_fmt
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/panicking.rs:75:14
   2: core::panicking::panic_const::panic_const_sub_overflow
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/panicking.rs:175:17
   3: uu_dd::tests::copy_buffer_does_not_touch_its_pages
             at ./src/dd.rs:1655:13
   4: uu_dd::tests::copy_buffer_does_not_touch_its_pages::{{closure}}
             at ./src/dd.rs:1632:46
   5: core::ops::function::FnOnce::call_once
             at /rust/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
   6: core::ops::function::FnOnce::call_once
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

https://github.com/uutils/coreutils/actions/runs/33278279221/job/99171233811?pr=14242

Start by running the copy_buffer_does_not_touch_its_pages test and inspect src/uu/dd/src/dd.rs around lines 1632 and 1655, where the backtrace identifies an overflowing subtraction. Reproduce the intermittent failure and trace the test inputs or page checks involved; done means the test passes reliably without the overflow panic.