Add SystemClock (#8239)

This PR adds a `SystemClock` trait for abstracting away the system
clock.

This allows us to swap out the real system clock with a
`FakeSystemClock` in the tests, thus allowing the fake passage of time.

We're using this in `Telemetry` to better mock the clock for testing
purposes.

Release Notes:

- N/A

crates/collab/src/tests/test_server.rs

@@ -10,6 +10,7 @@ use channel::{ChannelBuffer, ChannelStore};
 use client::{
     self, proto::PeerId, Client, Connection, Credentials, EstablishConnectionError, UserStore,
 };
+use clock::FakeSystemClock;
 use collab_ui::channel_view::ChannelView;
 use collections::{HashMap, HashSet};
 use fs::FakeFs;
@@ -163,6 +164,7 @@ impl TestServer {
             client::init_settings(cx);
         });
 
+        let clock = Arc::new(FakeSystemClock::default());
         let http = FakeHttpClient::with_404_response();
         let user_id = if let Ok(Some(user)) = self.app_state.db.get_user_by_github_login(name).await
         {
@@ -185,7 +187,7 @@ impl TestServer {
                 .user_id
         };
         let client_name = name.to_string();
-        let mut client = cx.update(|cx| Client::new(http.clone(), cx));
+        let mut client = cx.update(|cx| Client::new(clock, http.clone(), cx));
         let server = self.server.clone();
         let db = self.app_state.db.clone();
         let connection_killers = self.connection_killers.clone();