Python: Move test-modeling to API-graphs

Notice that although we loose the contrived examples in `test.py`, we do
gain support for real-world test-case construction, which seems worth
the tradeoff.

Author: RasmusWL

python/ql/lib/semmle/python/filters/Tests.qll

@@ -1,14 +1,15 @@
 import python
+private import semmle.python.ApiGraphs
 
 abstract class TestScope extends Scope { }
 
-// don't extend Class directly to avoid ambiguous method warnings
-class UnitTestClass extends TestScope {
+class UnitTestClass extends TestScope, Class {
   UnitTestClass() {
-    exists(ClassValue cls | this = cls.getScope() |
-      cls.getABaseType+() = Module::named("unittest").attr(_)
-      or
-      cls.getABaseType+().getName().toLowerCase() = "testcase"
+    exists(API::Node testCaseClass, string testCaseString |
+      testCaseString.matches("%TestCase") and
+      testCaseClass = any(API::Node mod).getMember(testCaseString)
+    |
+      this.getParent() = testCaseClass.getASubclass*().getAnImmediateUse().asExpr()
     )
   }
 }

python/ql/test/library-tests/filters/tests/Filter.expected

@@ -1,8 +1,13 @@
-| test.py:4:1:4:23 | Class MyTest |
-| test.py:6:5:6:21 | Function test_1 |
-| test.py:9:5:9:21 | Function test_2 |
 | test_foo.py:3:1:3:15 | Function test_foo |
 | unittest_test.py:3:1:3:33 | Class FooTest |
 | unittest_test.py:4:5:4:25 | Function test_valid |
 | unittest_test.py:7:1:7:49 | Class FunctionFooTest |
 | unittest_test.py:8:5:8:25 | Function test_valid |
+| unittest_test.py:11:1:11:50 | Class AsyncTest |
+| unittest_test.py:12:11:12:31 | Function test_valid |
+| unittest_test.py:17:1:17:45 | Class MyDjangoUnitTest |
+| unittest_test.py:18:5:18:25 | Function test_valid |
+| unittest_test.py:23:1:23:56 | Class MyFlaskUnitTest |
+| unittest_test.py:24:5:24:25 | Function test_valid |
+| unittest_test.py:29:1:29:59 | Class MyTornadoUnitTest |
+| unittest_test.py:30:5:30:25 | Function test_valid |