dd: fix flaky test_final_stats_unspec

[BenWiederhake]

If the first four decimal digits are zero, GNU dd elides them altogether. Here's an execution on my PC:

$ for i in $(seq 20000); do LC_ALL=C gnu_dd if=/dev/null of=/dev/null \
    2>&1; done | grep copied | grep -E ' [0-9]e'
0 bytes copied, 1e-05 s, 0 B/s
0 bytes copied, 9e-06 s, 0.0 kB/s

Our implementation conforms to this, resulting in the following CI flake:

---- test_dd::test_final_stats_unspec stdout ----
run: D:\a\coreutils\coreutils\target\x86_64-pc-windows-gnu\debug\coreutils.exe dd
thread 'test_dd::test_final_stats_unspec' panicked at 'Stderr does not match regex:
0+0 records in
0+0 records out
0 bytes copied, 8e-05 s, 0.0 B/s
', tests\by-util\test_dd.rs:280:10
stack backtrace:
   0: rust_begin_unwind
             at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library\std\src/panicking.rs:578:5

Of course, this is just an overly strict regex in the test. This was a one-in-tenthousand flaky test.

EDIT: Fun fact, I discovered this because it flaked in #5999 (comment) .