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.lock

@@ -1103,6 +1103,7 @@ dependencies = [
  "brotli",
  "chrono",
  "criterion",
+ "deltachat-time",
  "deltachat_derive",
  "email",
  "encoded-words",
@@ -1223,6 +1224,10 @@ dependencies = [
  "yerpc",
 ]
 
+[[package]]
+name = "deltachat-time"
+version = "1.0.0"
+
 [[package]]
 name = "deltachat_derive"
 version = "2.0.0"