Add test for CommonMark output

Author: mattt

Tests/EndToEndTests/GenerateSubcommandTests.swift

@@ -2,6 +2,47 @@ import class Foundation.Bundle
 import XCTest
 
 final class GenerateSubcommandTests: XCTestCase {
+    func testCommonMark() 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: [
+                            "generate",
+                            "--module-name", "SwiftDoc",
+                            "--format", "commonmark",
+                            "--output", outputDirectory.path,
+                            "Sources"
+                        ]
+        ) { result in
+            XCTAssertEqual(result.terminationStatus, EXIT_SUCCESS)
+            XCTAssertEqual(result.output, "")
+            XCTAssertEqual(result.error, "")
+
+            do {
+                let commonmark = try String(contentsOf: outputDirectory.appendingPathComponent("Home.md"))
+                XCTAssertTrue(commonmark.contains("# Types"))
+            }
+
+            do {
+                let commonmark = try String(contentsOf: outputDirectory.appendingPathComponent("_Sidebar.md"))
+                XCTAssertTrue(commonmark.contains("<summary>Types</summary>"))
+            }
+
+            do {
+                let commonmark = try String(contentsOf: outputDirectory.appendingPathComponent("_Footer.md"))
+                XCTAssertTrue(commonmark.contains("[swift-doc](https://github.com/SwiftDocOrg/swift-doc)"))
+            }
+
+            do {
+                let contents = try FileManager.default.contentsOfDirectory(at: outputDirectory, includingPropertiesForKeys: [.isDirectoryKey], options: [.skipsHiddenFiles])
+                let subdirectories = contents.filter { $0.hasDirectoryPath }
+                XCTAssertEqual(subdirectories.count, 0, "output should not contain any subdirectories")
+            }
+        }
+    }
+
     func testHTML() throws {
         let command = Bundle.productsDirectory.appendingPathComponent("swift-doc")
         let outputDirectory = try temporaryDirectory()