fix(test): do not fail test for filesystem race (#8643)

evidence:
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8632/10287641784/index.html#suites/0e58fb04d9998963e98e45fe1880af7d/c7a46335515142b/

pageserver/src/statvfs.rs

@@ -56,7 +56,6 @@ impl Statvfs {
 }
 
 pub mod mock {
-    use anyhow::Context;
     use camino::Utf8Path;
     use regex::Regex;
     use tracing::log::info;
@@ -135,14 +134,30 @@ pub mod mock {
             {
                 continue;
             }
-            total += entry
-                .metadata()
-                .with_context(|| format!("get metadata of {:?}", entry.path()))?
-                .len();
+            let m = match entry.metadata() {
+                Ok(m) => m,
+                Err(e) if is_not_found(&e) => {
+                    // some temp file which got removed right as we are walking
+                    continue;
+                }
+                Err(e) => {
+                    return Err(anyhow::Error::new(e)
+                        .context(format!("get metadata of {:?}", entry.path())))
+                }
+            };
+            total += m.len();
         }
         Ok(total)
     }
 
+    fn is_not_found(e: &walkdir::Error) -> bool {
+        let Some(io_error) = e.io_error() else {
+            return false;
+        };
+        let kind = io_error.kind();
+        matches!(kind, std::io::ErrorKind::NotFound)
+    }
+
     pub struct Statvfs {
         pub blocks: u64,
         pub blocks_available: u64,