Add a check to make sure runtime graphs are complete

We need to make sure Microsoft.NETCore.App.deps.json has at least one
runtime fallback graph. Otherwise, it is possible that .NET will fail to
find assets placed in the RID-fallback graph.

For more details, see dotnet/runtime#78563

runtime-fallback-graph/test.json

@@ -0,0 +1,11 @@
+{
+  "name": "runtime-fallback-graph",
+  "enabled": true,
+  "requiresSdk": false,
+  "version": "3.1",
+  "versionSpecific": false,
+  "type": "bash",
+  "cleanup": true,
+  "ignoredRIDs":[
+  ]
+}

runtime-fallback-graph/test.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+
+# Check runtime fallback graphs are present in the shared framework
+
+set -euo pipefail
+set -x
+
+dotnet_dir="$(../dotnet-directory --home "$1")"
+
+# print for  debugging
+find "${dotnet_dir}" -iname Microsoft.NETCore.App.deps.json
+
+while IFS= read -r -d '' file; do
+    jq '.runtimes' "$file"
+    length=$(jq '.runtimes | length' "$file")
+    if [[ $length == 0 ]]; then
+        echo "Missing .runtimes section in $file"
+        exit 1
+    fi
+done < <(find "${dotnet_dir}" -iname Microsoft.NETCore.App.deps.json -print0)