add a dummy package to test diagnostics

Author: tayloraswift

TestPackages/diagnostics/Package.swift

@@ -0,0 +1,12 @@
+// swift-tools-version:6.0
+import PackageDescription
+
+let package:Package = .init(name: "Swift Unidoc Test Modules",
+    products:
+    [
+        .library(name: "A", targets: ["A"]),
+    ],
+    targets:
+    [
+        .target(name: "A"),
+    ])

TestPackages/diagnostics/Sources/A/A.swift

@@ -0,0 +1,3 @@
+# ``/A``
+
+Here is a link to a symbol that does not exist: ``/B [ignore when: Linux]``.