tests/llvm: require cmake 3.11

This is a weird but working way to skip this test from running on
bionic (where it should not run).

test cases/frameworks/15 llvm/test.json

@@ -12,5 +12,8 @@
       ]
     }
   },
-  "skip_on_jobname": ["azure", "cygwin"]
+  "skip_on_jobname": ["azure", "cygwin"],
+  "tools": {
+    "cmake": ">=3.11"
+  }
 }