Switch getUnixTimestamp to cross platform code

[Dfte]

This PR switches the following line of code from the getUnixTime function:

return datetime.fromtimestamp(getUnixTime(int(str(self.fields[key]))))

To a cross platform independant oneliner:

return datetime(1601, 1, 1) + timedelta(microseconds=int(str(self.fields[key])) // 10)

It was mentionned as an issue on NetExec for people using it on Windows, which would be the same for people using Impacket on Windows:

Pennyw0rth/NetExec#827

And it mitigates it:

[gabrielg5]

yeah, if Data value is 0 it ends ups passing a negative value to datetime.fromtimestamp that could end up failing on windows systems.

Thinking if we should get rid of the (now) unused getUnixTime function definition in tsts.py.
But we can check it in another clean up ticket, also taking care of the many definitions of that same function all over the project

Merging, then, thanks!!