autotools: show test failure diff

[t-8ch]

This helps investigating issues, especially spurious errors only happening during CI like the failure of "hardlink/options-maximum-size-8192"

[t-8ch]

And we even luckily? hit the hardlink/options failure:

--- /home/runner/work/util-linux/util-linux/util-linux-2.38.602-9fbb0/tests/expected/hardlink/options-maximum-size-8192	2022-11-30 05:11:46.507640311 +0000
+++ /home/runner/work/util-linux/util-linux/util-linux-2.38.602-9fbb0/_build/sub/tests/output/hardlink/options-maximum-size-8192	2022-11-30 05:19:18.399075246 +0000
@@ -4,7 +4,7 @@
 Files:                    26
 Linked:                   18 files
 Compared:                 0 xattrs
-Compared:                 23 files
+Compared:                 22 files
 Saved:                    144 KiB
 Duration: [Redacted]
 dir-1/sdir-1/file-a-1	5	8192	1540236330	644
}}}-diff