testsuite: make basis xname oracle deterministic

[steadytao]

Addressing master CI failures.

Summary

• replace the timing-sensitive FIFO oracle in basis-xname-traversal
• trace the exact basedir and relpath passed to secure_basis_open()
• preserve the malicious sender injection and forced-protocol coverage
• remove platform-dependent retries and FIFO rendezvous timing

The existing FIFO oracle intermittently reports a vacuous run on NetBSD even though the same test passes in later runs without relevant source changes. The instrumented receiver observes the security decision directly therefore ../secret must not reach the basis open and linkdest/secret must be recorded.