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/context.rs

@@ -1217,7 +1217,7 @@ pub fn get_version_str() -> &'static str {
 
 #[cfg(test)]
 mod tests {
-    use std::time::{Duration, SystemTime};
+    use std::time::Duration;
 
     use anyhow::Context as _;
     use strum::IntoEnumIterator;
@@ -1234,7 +1234,7 @@ mod tests {
     use crate::mimeparser::SystemMessage;
     use crate::receive_imf::receive_imf;
     use crate::test_utils::{get_chat_msg, TestContext};
-    use crate::tools::create_outgoing_rfc724_mid;
+    use crate::tools::{create_outgoing_rfc724_mid, SystemTime};
 
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn test_wrong_db() -> Result<()> {