lib/std/fs/test.zig: Some filesystems support 8 EiB files

Btrfs at least supports 16 EiB files (limited in practice to 8EiB by the
Linux VFS code which uses signed 64-bit offsets).  So fix the fs.zig test
case to expect either a FileTooBig or success from truncating a file to
8EiB.  And test that beyond that size the offset is interpreted as a
negative number.

Fixes #24242

Author: rootbeer

lib/std/fs/test.zig

@@ -1435,14 +1435,17 @@ test "setEndPos" {
     try testing.expectEqual(0, try f.preadAll(&buffer, 0));
 
     // Invalid file length should error gracefully. Actual limit is host
-    // and file-system dependent, but 1PB should fail most everywhere.
-    // Except MacOS APFS limit is 8 exabytes.
+    // and file-system dependent, but 1PB should fail on filesystems like
+    // EXT4 and NTFS.  But XFS or Btrfs support up to 8EiB files.
     f.setEndPos(0x4_0000_0000_0000) catch |err| if (err != error.FileTooBig) {
         return err;
     };
 
-    try testing.expectError(error.FileTooBig, f.setEndPos(std.math.maxInt(u63))); // Maximum signed value
+    f.setEndPos(std.math.maxInt(u63)) catch |err| if (err != error.FileTooBig) {
+        return err;
+    };
 
+    try testing.expectError(error.FileTooBig, f.setEndPos(std.math.maxInt(u63) + 1));
     try testing.expectError(error.FileTooBig, f.setEndPos(std.math.maxInt(u64)));
 }