tests: posix: nanosleep: round up to the nearest microsecond

Here, we include some addtional tests for durations that have
sub-microsecond components.

1ns => k_busy_wait(0). Round to 1us.
1us + 1ns => k_busy_wait(1us). Round to 2us.
1s + 1ns => k_busy_wait(1000000us). Round to 1000001us.
1s + 1us + 1ns => k_busy_wait(1000001us). Round to 1000002us.

Fixes #28483

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>
2 files changed