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.

deltachat-time/src/lib.rs

@@ -0,0 +1,35 @@
+#![allow(missing_docs)]
+
+use std::sync::RwLock;
+use std::time::{Duration, SystemTime};
+
+static SYSTEM_TIME_SHIFT: RwLock<Duration> = RwLock::new(Duration::new(0, 0));
+
+/// Fake struct for mocking `SystemTime::now()` for test purposes. You still need to use
+/// `SystemTime` as a struct representing a system time.
+pub struct SystemTimeTools();
+
+impl SystemTimeTools {
+    pub const UNIX_EPOCH: SystemTime = SystemTime::UNIX_EPOCH;
+
+    pub fn now() -> SystemTime {
+        return SystemTime::now() + *SYSTEM_TIME_SHIFT.read().unwrap();
+    }
+
+    /// Simulates a system clock forward adjustment by `duration`.
+    pub fn shift(duration: Duration) {
+        *SYSTEM_TIME_SHIFT.write().unwrap() += duration;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        SystemTimeTools::shift(Duration::from_secs(60));
+        let t = SystemTimeTools::now();
+        assert!(t > SystemTime::now());
+    }
+}