Add a check to make sure runtime graphs are complete

We need to make sure Microsoft.NETCore.App.deps.json has a complete
runtime fallback graph. Otherwise, it is possible that .NET will fail to
find assets placed in paths based on 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 +1,33 @@
+#!/usr/bin/env bash
+
+# Check runtime fallback graphs are present in the shared framework
+
+set -euo pipefail
+set -x
+
+dotnet_dir="$(../dotnet-directory --home "$1")"
+portable_rid="$(../runtime-id --portable)"
+non_portable_rid="$(../runtime-id)"
+
+# print for  debugging
+find "${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 $file"
+        exit 1
+    fi
+    # quoting here is a bit strange, but it's basically ".runtimes[\"" "$non_portable_rid" "\"] | length" without spaces
+    length=$(jq ".runtimes[\"""$non_portable_rid""\"] | length" "$file")
+    if [[ $length == 0 ]]; then
+        echo "Missing runtimes[$non_portable_rid] section in $file"
+        exit 1
+    fi
+    fallback_graph=$(jq ".runtimes[\"""$non_portable_rid""\"]" "$file")
+    echo "$fallback_graph" | grep "$portable_rid"
+    echo "$fallback_graph" | grep "linux"
+    echo "$fallback_graph" | grep "unix"
+    echo "$fallback_graph" | grep "base"
+done < <(find "${dotnet_dir}" -iname Microsoft.NETCore.App.deps.json -print0)