tests/unit: add hierarchical_barrier correctness test

[bcmIntc]

Summary

• Adds test/unit/hierarchical_barrier.c, a four-subtest correctness suite for shmem_barrier_all() targeting the hierarchical barrier introduced by
  --enable-hierarchical-barrier in the SOS library. The test is also valid and meaningful on non-hierarchical builds.
• write_visibility: each PE puts its PE number to its right neighbor then barriers; verifies the remote write is visible on return. Catches barriers that return before
  all puts are globally visible.
• successive_barriers: 8 back-to-back put+barrier+verify rounds per iteration. Stresses pSync slot reset — specifically the race where a signal from call N+1 arrives
  before call N's slot is cleared, causing a hang or spurious early completion.
• asymmetric_arrival: even PEs spin before entering the barrier, odd PEs enter immediately. Verifies correct completion under staggered arrival, which exercises the
  tree gather path under real timing skew.
• atomic_counter: all PEs atomically increment a counter on PE 0 each iteration; PE 0 verifies the cumulative total equals npes * loop after each barrier. Catches lost
  updates that would indicate CPU/NIC coherency hazards or barrier ordering failures.

Test plan

• Build without --enable-hierarchical-barrier and confirm all four subtests pass (baseline correctness)
• Build with --enable-hierarchical-barrier and confirm all four subtests pass
• Run at small scale (2 PEs, same node) — exercises the single-node degenerate path
• Run at PPN >= 64, 2+ nodes — exercises the full three-phase hierarchical path
• Run with -l 1000 to stress pSync reset under rapid successive calls
• Run with -v to confirm per-subtest pass/fail output is readable