Judge tests that read another repository against what the check clones, never the desktop's checkout (issue 432)

Tests that read ../../../mesh-catalog or ../../../mesh-host gave a verdict
that depended on what sat beside the checkout: a stale or dirty sibling
failed them on a desktop, and a missing one skipped them unseen. They now
read the clone the build seat puts in MESH_CHECK_BESIDE, failing when it is
absent there, and elsewhere a copy captured at a named commit.

The skip had hidden that the builder test read a module retired by ADR 0190.
The systemd reading test no longer counts the machine's own environment.d.

testdata/beside/mesh-catalog/modules/zsh-syntax-highlighting/module.json.captured

@@ -0,0 +1,21 @@
+{
+  "module": "zsh-syntax-highlighting",
+  "version": "1",
+  "capabilities": [
+    "package-manager"
+  ],
+  "shell": [
+    {
+      "for": "zsh",
+      "slot": "last",
+      "code": "# Syntax highlighting of the command line, from the distribution's package. Last, as upstream asks:\n# it wraps every widget defined before it.\n[[ ! -f /usr/share/zsh/plugins/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh ]] || source /usr/share/zsh/plugins/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh\n"
+    }
+  ],
+  "resources": [
+    {
+      "id": "package",
+      "type": "package",
+      "package": "zsh-syntax-highlighting"
+    }
+  ]
+}