Judge the SDK conformance fixtures against the SDK the controller pins, never a desktop's checkout (issue 449)

A check clones mesh-sdk beside the controller at the commit go.mod pins; elsewhere
the test reads a copy captured at that commit, held to go.mod. A missing clone fails
instead of skipping.

internal/beside/beside.go

@@ -1,5 +1,5 @@
 // Package beside is where a test finds another repository of the mesh it reads: the catalogue's manifests,
-// the node-engine's genesis template (novox/hq issue 432).
+// the node-engine's genesis template (novox/hq issue 432), the SDK's conformance fixtures (issue 449).
 //
 // A test used to read the checkout beside this one, `../../../mesh-catalog`, so its verdict depended on
 // whatever sat on the machine running it: a stale or dirty checkout failed it on a desktop, and where none
@@ -10,7 +10,8 @@
 //     test judges against those clones, so agreement with the other repository is checked where
 //     `mesh/repo-check` runs; a repository missing there fails the test, never skips it. Which clones a
 //     check gets is chosen from the inventory: mesh-catalog by the source of the `nats` module, mesh-host
-//     by the source of `mesh-host`. In a mesh where either module has no source repository nothing is
+//     by the source of `mesh-host`, and mesh-sdk as the controller's sibling at the commit the controller's
+//     go.mod pins. In a mesh where either module has no source repository nothing is
 //     cloned, and these tests fail loudly with "not beside this check": a cause in the setup, not in the
 //     change. And in a delivery group that holds a mesh-catalog pull request, these tests read the
 //     catalogue's main, not the group's head.