[3.12] gh-126909: Fix running xattr tests on systems with lower limits (GH-126930) (#126964)

gh-126909: Fix running xattr tests on systems with lower limits (GH-126930)

Modify the extended attribute tests to write fewer and smaller extended
attributes, in order to fit within filesystems with total xattr limit
of 1 KiB (e.g. ext4 with 1 KiB blocks).  Previously, the test would
write over 2 KiB, making it fail with ENOSPC on such systems.
(cherry picked from commit 2c0a21c1aad65ab8362491acf856eb574b1257ad)

Co-authored-by: Michał Górny <mgorny@gentoo.org>