testsuite: mark fstest XFAIL

Problem: tests/kern/t31 (fstest) fails on newer ubuntu 22.04's
kernel 6.9.3 but not on yocto's 6.6.45.

Since ubuntu-latest is currently 22.04 in CI, mark XFAIL
until we can get to the root cause.

See chaos#103

Author: garlick

tests/kern/Makefile.am

@@ -28,7 +28,7 @@ TESTS = t05 t06 t07 t12 t13 t14 t15 t16 t17 t18 t19 t22 \
 	t23 t24 t25 t26 t27 t28 t29 t30 t31 t32 t33 \
 	t34 t35 t36 t37 t38 t39 t40 t41 t42 t43 t44 t45 t46 t47
 
-XFAIL_TESTS = t19 t23 t37 t43 t47 t48 t49
+XFAIL_TESTS = t19 t23 t31 t37 t43 t47 t48 t49
 
 CLEANFILES = *.out *.diff *.diod *.postmark *.dbench