Add tests for diagram subcommand

Author: mattt

Tests/EndToEndTests/DiagramSubcommandTests.swift

@@ -0,0 +1,21 @@
+import XCTest
+
+final class DiagramSubcommandTests: XCTestCase {
+    func testStandardOutput() throws {
+        let command = Bundle.productsDirectory.appendingPathComponent("swift-doc")
+
+        let outputDirectory = try temporaryDirectory()
+        defer { try? FileManager.default.removeItem(at: outputDirectory) }
+
+        try Process.run(command: command,
+                        arguments: [
+                            "diagram",
+                            "Sources"
+                        ]
+        ) { result in
+            XCTAssertEqual(result.terminationStatus, EXIT_SUCCESS)
+            XCTAssertEqual(result.output?.starts(with: "digraph {"), true)
+            XCTAssertEqual(result.error, "")
+        }
+    }
+}