Python: Add import * test

This test shows off a few things:

- transitive chains of `import *`
- multiple modules exporting the same name (to test for cross-talk)

python/ql/test/experimental/dataflow/import-star/deux.py

@@ -0,0 +1,2 @@
+from trois import *
+print(foo)

python/ql/test/experimental/dataflow/import-star/global.expected

@@ -0,0 +1,3 @@
+moduleVariables
+reads
+writes

python/ql/test/experimental/dataflow/import-star/global.ql

@@ -0,0 +1,12 @@
+import python
+import semmle.python.dataflow.new.DataFlow
+
+query DataFlow::Node moduleVariables() { result instanceof DataFlow::ModuleVariableNode }
+
+query predicate reads(DataFlow::Node fromNode, DataFlow::Node toNode) {
+  fromNode.(DataFlow::ModuleVariableNode).getARead() = toNode
+}
+
+query predicate writes(DataFlow::Node fromNode, DataFlow::Node toNode) {
+  fromNode = toNode.(DataFlow::ModuleVariableNode).getAWrite()
+}

python/ql/test/experimental/dataflow/import-star/one.py

@@ -0,0 +1 @@
+from two import *

python/ql/test/experimental/dataflow/import-star/test1.py

@@ -0,0 +1,2 @@
+from one import *
+print(foo)

python/ql/test/experimental/dataflow/import-star/test2.py

@@ -0,0 +1,2 @@
+from un import *
+print(foo)

python/ql/test/experimental/dataflow/import-star/three.py

@@ -0,0 +1 @@
+foo = 5

python/ql/test/experimental/dataflow/import-star/trois.py

@@ -0,0 +1 @@
+foo = 5

python/ql/test/experimental/dataflow/import-star/two.py

@@ -0,0 +1,2 @@
+from three import *
+print(foo)

python/ql/test/experimental/dataflow/import-star/un.py

@@ -0,0 +1 @@
+from deux import *