User per-guest SSH master sockets to prevent re-use (#3127)

It seems that under some conditions, using same socket path for SSH
master processes may lead to SSH commands to operate over incorrect and
unexpected guest without noticing. Using per-guest paths should prevent
such a race condition.

The problem was revealed by `/tests/prepare/multihost` test constantly
and reliably failing in Github CI, as all `prepare` phases were
connecting to the very same guest.

tmt/steps/provision/__init__.py

@@ -1392,7 +1392,7 @@ def _ssh_master_socket_path(self) -> Path:
         # Use '/run/user/uid' if it exists, 'temp dir' otherwise.
         run_dir = Path(f"/run/user/{os.getuid()}")
         socket_dir = run_dir if run_dir.is_dir() else Path(tempfile.mkdtemp())
-        return socket_dir / "tmt"
+        return socket_dir / f'tmt-{self.pathless_safe_name}'
 
     @property
     def _ssh_options(self) -> Command: