adds a base symbolgraph for Snippets test fixture

Supports fixing an issue confusing snippets as both "main" and "extension"
symbolgraphs for a particular module. The fix in swift-docc-symbolkit
fails this specific fixture since it doesn't currently have a "main"
graph, even empty as this one, to extend from. With this in place, the
tests fully pass for properly resolving snippets symbolgraphs reliably.

Author: heckj

Tests/SwiftDocCTests/Test Bundles/Snippets.docc/Test.symbols.json

@@ -0,0 +1,16 @@
+{
+    "metadata": {
+        "formatVersion": {
+            "major": 0,
+            "minor": 6,
+            "patch": 0
+        },
+        "generator": "SwiftPM"
+    },
+    "module": {
+        "name": "Snippets",
+        "platform": {}
+    },
+    "symbols": [],
+    "relationships": []
+}