feat: take the host probe lock per run

core_probe takes probe.lock itself around one execution, load check included, and releases it when the run ends. A campaign pass never holds it between runs, so two passes interleave. RETROBIOS_PROBE_LOCK names another file, RETROBIOS_PROBE_LOCK_HELD says the caller holds it. Tests use their own lock file.

tests/__init__.py

@@ -16,6 +16,11 @@ import os
 from pathlib import Path
 
 os.environ.setdefault("RETROBIOS_OFFLINE", "1")
+# A test never waits on, nor blocks, the measurements running on the host.
+os.environ.setdefault(
+    "RETROBIOS_PROBE_LOCK",
+    str(Path(__file__).resolve().parent.parent / "tmp" / "tests" / "probe.lock"),
+)
 
 (Path(__file__).resolve().parent.parent / "tmp" / "tests").mkdir(
     parents=True, exist_ok=True