lib/libexecinfo/sigtramp_test: expect failure on all architectures

[ngie]

Summary

The test started failing recently on amd64 due to 64a73d1b6dc9. Mark the
test as an expected failure until LLVM's libunwind is enhanced on
FreeBSD to represent missing functionality per DWARF.

PR: 289096
MFC after: 3 days

Test Plan

On amd64:

% kyua test -k /usr/tests/lib/libexecinfo/Kyuafile 
backtrace_test:backtrace_fmt_basic  ->  passed  [0.004s]
sigtramp_test:test_backtrace_sigtramp  ->  expected_failure: Bug 289096: test fails on all archs (libunwind provided by clang lacks functionality): /usr/src/lib/libexecinfo/tests/sigtramp_test.c:30: n > 1 not met  [0.003s]

Results file id is usr_tests_lib_libexecinfo.20261020-155757-369199
Results saved to /home/ngie/.kyua/store/results.usr_tests_lib_libexecinfo.20261020-155757-369199.db

2/2 passed (0 broken, 0 failed, 0 skipped)