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/ephemeral.rs

@@ -67,7 +67,7 @@ use std::collections::BTreeSet;
 use std::convert::{TryFrom, TryInto};
 use std::num::ParseIntError;
 use std::str::FromStr;
-use std::time::{Duration, SystemTime, UNIX_EPOCH};
+use std::time::{Duration, UNIX_EPOCH};
 
 use anyhow::{ensure, Result};
 use async_channel::Receiver;
@@ -85,7 +85,7 @@ use crate::message::{Message, MessageState, MsgId, Viewtype};
 use crate::mimeparser::SystemMessage;
 use crate::sql::{self, params_iter};
 use crate::stock_str;
-use crate::tools::{duration_to_str, time};
+use crate::tools::{duration_to_str, time, SystemTime};
 
 /// Ephemeral timer value.
 #[derive(Debug, PartialEq, Eq, Copy, Clone, Serialize, Deserialize)]