fix preview2_stdin test on Asahi Linux (#13126)

This has test has been failing on my Asahi Ubuntu Linux (ARM64) machine since
the first time I ran it, but I never got around to actually addressing it until
now.  Seems that the `slop` factor in this test was just _one_ byte too small.

Not sure why this never broke on CI; might be specific to Apple hardware,
e.g. 16K page sizes.

Author: dicej

tests/all/cli_tests.rs

@@ -1044,7 +1044,10 @@ fn preview2_stdin() -> Result<()> {
     // helper thread depends on how much the OS buffers for us. For now give
     // some some slop and assume that OSes are unlikely to buffer more than
     // that.
-    let slop = 256 * 1024;
+    //
+    // Note that 256 * 1024 is _one_ byte too small on Asahi Linux (possibly
+    // related to 16K page sizes?), hence the `+ 1` here:
+    let slop = 256 * 1024 + 1;
     for amt in [0, 100, 100_000] {
         let written = count_up_to(amt)?;
         assert!(written < slop + amt, "wrote too much {written}");