(testsuite) Use kern instead of fs to test sysctl completion.

FreeBSD does not have fs.* (has vfs.* instead).  Linux has kernel.*,
FreeBSD kern.*.

test/lib/completions/sysctl.exp

@@ -11,7 +11,9 @@ proc teardown {} {
 setup
 
 
-assert_complete_any "sysctl fs"
+if {[assert_exec {sysctl -N -a 2>/dev/null | grep ^kern | sort -u} values]} {
+    assert_complete $values "sysctl kern"
+}; # if
 
 
 sync_after_int