selftests: mptcp: drop nanoseconds width specifier

selftests: mptcp: drop nanoseconds width specifier

Using the format specifier +%s%3N with GNU date is honoured, and only
prints 3 digits of the nanoseconds portion of the seconds since epoch,
which corresponds to the milliseconds.

The uutils implementation of date currently does not honour this, and
always prints all 9 digits. This is a known issue [1], but can be worked
around by adapting this test to use nanoseconds instead of microseconds,
and then divide it by 1e6.

This fix is similar to what has been done on systemd side [2], and it is
needed to run the selftests on Ubuntu 26.04, containing uutils 0.8.0.

Note that the Fixes tag is there even if this patch doesn't fix an issue
in the kernel selftests, but it is useful for those using uutils 0.8.0.

Fixes: 048d19d444be ("mptcp: add basic kselftest for mptcp")
Cc: stable@vger.kernel.org
Link: https://github.com/uutils/coreutils/issues/11658
Link: https://github.com/systemd/systemd/pull/41627
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260515-net-mptcp-misc-fixes-7-1-rc4-v2-6-701e96419f2f@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>