C++: Add 'pthread_create' test with missing flow.

cpp/ql/test/library-tests/dataflow/external-models/sinks.expected

@@ -8,3 +8,4 @@
 | test.cpp:29:10:29:11 | y3 | test-sink |
 | test.cpp:33:10:33:11 | z2 | test-sink |
 | test.cpp:36:10:36:11 | z3 | test-sink |
+| test.cpp:48:14:48:14 | x | test-sink |

cpp/ql/test/library-tests/dataflow/external-models/sources.expected

@@ -1,5 +1,6 @@
 | asio_streams.cpp:87:34:87:44 | read_until output argument | remote |
 | test.cpp:10:10:10:18 | call to ymlSource | local |
+| test.cpp:56:8:56:16 | call to ymlSource | local |
 | windows.cpp:22:15:22:29 | *call to GetCommandLineA | local |
 | windows.cpp:34:17:34:38 | *call to GetEnvironmentStringsA | local |
 | windows.cpp:39:36:39:38 | GetEnvironmentVariableA output argument | local |

cpp/ql/test/library-tests/dataflow/external-models/test.cpp

@@ -35,3 +35,26 @@ void test() {
 	int z3 = ymlStepGenerated_with_body(x, 0);
 	ymlSink(z3); // clean
 }
+
+struct S {
+	int x;
+};
+
+using pthread_t = unsigned long;
+using pthread_attr_t = void*;
+
+void *myThreadFunction(void *arg) {
+    S* s = (S *)arg;
+		ymlSink(s->x); // $ MISSING: ir
+    return nullptr;
+}
+
+int pthread_create(pthread_t *thread, const pthread_attr_t * attr, void *(*start_routine)(void*), void *arg);
+
+int test_pthread_create() {
+	S s;
+	s.x = ymlSource();
+
+	pthread_t threadId;
+	pthread_create(&threadId, nullptr, myThreadFunction, (void *)&s);
+}