Fix test that became flaky after recent snippets change (#1308)

Depending on which symbol was _first_ in the dictionary of symbols, the entire symbol graph was categorized as either a snippet symbol graph or a regular symbol graph.
However, real symbol graph files don't mix snippets and framework symbols like that. This fixes the test to don't mix snippets with real symbols.
It also stops adding module nodes inside the symbol graph, because that also doesn't match real symbol graph files.

Author: d-ronnqvist

Tests/SwiftDocCTests/Infrastructure/DocumentationContext/DocumentationContextTests.swift

@@ -4935,7 +4935,11 @@ let expected = """
     func testContextDoesNotRecognizeNonOverloadableSymbolKinds() async throws {
         enableFeatureFlag(\.isExperimentalOverloadedSymbolPresentationEnabled)
 
-        let nonOverloadableKindIDs = SymbolGraph.Symbol.KindIdentifier.allCases.filter { !$0.isOverloadableKind }
+        let nonOverloadableKindIDs = SymbolGraph.Symbol.KindIdentifier.allCases.filter {
+            !$0.isOverloadableKind &&
+            !$0.isSnippetKind      && // avoid mixing snippets with "real" symbols
+            $0 != .module             // avoid creating multiple modules
+        }
         // Generate a 4 symbols with the same name for every non overloadable symbol kind
         let symbols: [SymbolGraph.Symbol] = nonOverloadableKindIDs.flatMap { [
             makeSymbol(id: "first-\($0.identifier)-id",  kind: $0, pathComponents: ["SymbolName"]),