tests/unit: add FEMU pqueue regression coverage

[shining-b-02]

Summary

• add a GLib unit-test target for the FEMU priority queue
• verify min-heap insertion and pop ordering
• cover the pre-updated priority scenario fixed by repair pqueue using current heap relation #192
• cover random removal when the replacement must bubble up

Motivation

Recent fixes changed how pqueue_change_priority() and pqueue_randpop() restore the heap, but the priority queue had no focused unit coverage. These tests exercise both repair directions and assert the heap invariant after each operation.

Testing

• configured a minimal aarch64-softmmu build on macOS
• built tests/unit/test-femu-pqueue with Ninja
• ran the Meson unit test: 3 subtests passed
• ran scripts/checkpatch.pl: 0 errors, 0 warnings
• ran git diff --check successfully

The PR is left as a draft while upstream CI runs.