Assert that macOS on CI is high granularity

This raises the risk of a failure when there is no bug, but
hopefully only slightly, and it's a simple way to verify that the
snapshot `journey` test really is running fully in at least one
regularlly tested platform.

Author: EliahKagan

Cargo.lock

@@ -27,5 +27,6 @@ serde = { version = "1.0.114", optional = true, default-features = false, featur
 fastrand = { version = "2.1.0", default-features = false, features = ["std"] }
 
 [dev-dependencies]
-tempfile = "3.5.0"
 crossbeam-channel = "0.5.0"
+is_ci = "1.1.1"
+tempfile = "3.5.0"

gix-fs/Cargo.toml

@@ -64,5 +64,9 @@ fn has_granular_times(root: &Path) -> std::io::Result<bool> {
 
     // This could be wrongly false if a filesystem has very precise timings yet is ridiculously
     // fast. Then the `journey` test wouldn't run, though it could. But that's OK, and unlikely.
+    // However, for now, on CI, on macOS only, we assert the expectation of high granularity.
+    if cfg!(target_os = "macos") && is_ci::cached() {
+        assert_eq!(times.len(), n);
+    }
     Ok(times.len() == n)
 }