test: do not use nanoseconds width specifier in date command

test: do not use nanoseconds width specifier in date command

Using the format specifier +%s%6N with GNU date is honored, and only
prints 6 digits of the nanoseconds portion of the seconds since epoch.
The uutils implementation of date does not honor this, and always prints
all 9 digits. This is a known bug[1], but can be worked around by
adapting this test to use nanoseconds instead of microseconds.

[1] https://github.com/uutils/coreutils/issues/11658