feat: Mock SystemTime::now() for the tests

Add a new crate `deltachat_time` with a fake `struct SystemTimeTools` for mocking
`SystemTime::now()` for test purposes. One still needs to use `std::time::SystemTime` as a struct
representing a system time. I think such a minimalistic approach is ok -- even if somebody uses the
original `SystemTime::now()` instead of the mock by mistake, that could break only tests but not the
program itself. The worst thing that can happen is that tests using `SystemTime::shift()` and
checking messages timestamps f.e. wouldn't catch the corresponding bugs, but now we don't have such
tests at all which is much worse.

src/contact.rs

@@ -6,7 +6,7 @@ use std::convert::{TryFrom, TryInto};
 use std::fmt;
 use std::ops::Deref;
 use std::path::{Path, PathBuf};
-use std::time::{SystemTime, UNIX_EPOCH};
+use std::time::UNIX_EPOCH;
 
 use anyhow::{bail, ensure, Context as _, Result};
 use async_channel::{self as channel, Receiver, Sender};
@@ -36,7 +36,7 @@ use crate::sql::{self, params_iter};
 use crate::sync::{self, Sync::*};
 use crate::tools::{
     duration_to_str, get_abs_path, improve_single_line_input, strip_rtlo_characters, time,
-    EmailAddress,
+    EmailAddress, SystemTime,
 };
 use crate::{chat, stock_str};