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.

Cargo.toml

@@ -32,6 +32,7 @@ strip = true
 
 [dependencies]
 deltachat_derive = { path = "./deltachat_derive" }
+deltachat-time = { path = "./deltachat-time" }
 format-flowed = { path = "./format-flowed" }
 ratelimit = { path = "./deltachat-ratelimit" }
 
@@ -128,6 +129,7 @@ members = [
   "deltachat-rpc-server",
   "deltachat-ratelimit",
   "deltachat-repl",
+  "deltachat-time",
   "format-flowed",
 ]