crt: tests: update t_tsearch and mark it as XFAIL

Add logic to test how many nodes have been visited by a single call to `twalk`;
this allows to test that nodes were properly inserted and removed.

This change has uncovered a bug in `tdestroy` which causes this test to fail.

Add t_tsearch to testcases/Makefile.am, but mark it as XFAIL.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>