Standardise and add safety comments.

These will soon be required by a lint.

Bug: 290018030
Test: m rust
Change-Id: If62281a5eb2160061a11f28588b7a14e32ce265b
diff --git a/rialto/tests/test.rs b/rialto/tests/test.rs
index 98c935d..2dbd0cb 100644
--- a/rialto/tests/test.rs
+++ b/rialto/tests/test.rs
@@ -140,6 +140,7 @@
 
     // SAFETY: These are new FDs with no previous owner.
     let reader = unsafe { File::from_raw_fd(reader_fd) };
+    // SAFETY: These are new FDs with no previous owner.
     let writer = unsafe { File::from_raw_fd(writer_fd) };
 
     thread::spawn(|| {