Add test for Test module.

Check that Package.xctest are generated.

Author: kostiakoval

Fixtures/SwiftTesting/SingleTarget/Package.swift

@@ -0,0 +1,3 @@
+func ten() -> Int {
+    return 10
+}

Fixtures/SwiftTesting/SingleTarget/Sources/Foo/Foo.swift

@@ -0,0 +1,9 @@
+@testable import Foo
+import XCTest
+
+class SimpleGetTests: XCTestCase {
+
+    func testGetRequestStatusCode() {
+        XCTAssertEqual(ten(), 10)
+    }
+}

Fixtures/SwiftTesting/SingleTarget/Tests/Foo/Test.swift

@@ -303,4 +303,12 @@ class MiscellaneousTestCase: XCTestCase {
             XCTAssertEqual(output, "Goodbye\n")
         }
     }
+
+    func testModuleWithTests() {
+        fixture(name: "SwiftTesting/SingleTarget", file: #file, line: #line) { prefix in
+            XCTAssertBuilds(prefix)
+            XCTAssertDirectoryExists(prefix, ".build", "debug", "Package.xctest")
+            XCTAssertFileExists(prefix, ".build", "debug", "Foo.swiftmodule")
+        }
+    }
 }