Add data flowsources test

Author: atorralba

swift/ql/test/library-tests/dataflow/flowsources/FlowSources.expected

@@ -7,6 +7,8 @@
 | nsdata.swift:18:17:18:40 | call to init(contentsOf:) | external |
 | nsdata.swift:19:17:19:17 | call to init(contentsOf:options:) | external |
 | nsdata.swift:19:17:19:53 | call to init(contentsOf:options:) | external |
+| data.swift:18:20:18:20 | call to init(contentsOf:options:) | external |
+| data.swift:18:20:18:54 | call to init(contentsOf:options:) | external |
 | string.swift:56:21:56:21 | call to init(contentsOf:) | external |
 | string.swift:56:21:56:44 | call to init(contentsOf:) | external |
 | string.swift:57:21:57:21 | call to init(contentsOf:encoding:) | external |

swift/ql/test/library-tests/dataflow/flowsources/data.swift

@@ -0,0 +1,19 @@
+// --- stubs ---
+
+struct URL
+{
+	init?(string: String) {}
+}
+
+
+struct Data {
+    struct ReadingOptions : OptionSet { let rawValue: Int }
+	init(contentsOf: URL, options: ReadingOptions) {}
+}
+
+// --- tests ---
+
+func testData() {
+    let url = URL(string: "http://example.com/")
+    let data = try Data(contentsOf: url!, options: []) // SOURCE
+}