Fix clang 21 unterminated-string warnings in wolfTPM's SPDM unit test

[aidangarske]

Context

clang 21 warns -Wunterminated-string-initialization on byte plain[16] = "..." in wolfTPM src/spdm/unit_test.c (lines 2263 and 2299). The same pattern was already fixed in wolfSPDM's test/unit_test.c.

Origin

aidangarske/wolfTPM branch spdm-use-wolfspdm

What is needed

• Use unsized arrays or explicit byte lists so the initializer fits.

Blockers and dependencies

No response

Acceptance criteria

wolfTPM builds its SPDM unit test warning-free with clang 21.