tests/fs_file.rs: Explicitly drop temp file from cache

...in an attempt to stimulate both the `preadv2` direct from cache
codepath in addition to the punt to a threadpool uncached one.

tokio/tests/fs_file.rs

@@ -22,6 +22,26 @@ async fn basic_read() {
 
     assert_eq!(n, HELLO.len());
     assert_eq!(&buf[..n], HELLO);
+
+    // Drop the data from cache to stimulate uncached codepath on Linux (see preadv2 in file.rs)
+    #[cfg(target_os = "linux")]
+    {
+        use std::os::unix::io::AsRawFd;
+        nix::unistd::fsync(tempfile.as_raw_fd()).unwrap();
+        nix::fcntl::posix_fadvise(
+            tempfile.as_raw_fd(),
+            0,
+            0,
+            nix::fcntl::PosixFadviseAdvice::POSIX_FADV_DONTNEED,
+        )
+        .unwrap();
+    }
+
+    let mut file = File::open(tempfile.path()).await.unwrap();
+    let n = file.read(&mut buf).await.unwrap();
+
+    assert_eq!(n, HELLO.len());
+    assert_eq!(&buf[..n], HELLO);
 }
 
 #[tokio::test]