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/timesmearing.rs

@@ -79,11 +79,11 @@ impl SmearedTimestamp {
 
 #[cfg(test)]
 mod tests {
-    use std::time::SystemTime;
-
     use super::*;
     use crate::test_utils::TestContext;
-    use crate::tools::{create_smeared_timestamp, create_smeared_timestamps, smeared_time, time};
+    use crate::tools::{
+        create_smeared_timestamp, create_smeared_timestamps, smeared_time, time, SystemTime,
+    };
 
     #[test]
     fn test_smeared_timestamp() {