Fixed "test_*_path_exists" + CompilerDirectivesNode (GH-3619)

When test_assert_path_exists or test_fail_if_path_exists
was used on a function containing a CompilerDirectivesNode
it was inherited by that CompilerDirectivesNode. Therefore
you got misleading test failures if the path was in the
function but not within that CompilerDirectivesNode.

Cython/Compiler/ParseTreeTransforms.py

@@ -993,6 +993,10 @@ class InterpretCompilerDirectives(CythonTransform):
 
         old_directives = self.directives
         new_directives = dict(old_directives)
+        # test_assert_path_exists and test_fail_if_path_exists should not be inherited
+        # otherwise they can produce very misleading test failures
+        new_directives.pop('test_assert_path_exists', None)
+        new_directives.pop('test_fail_if_path_exists', None)
         new_directives.update(directives)
 
         if new_directives == old_directives:

tests/compile/tree_assertions.pyx

+# mode: compile
+
+# This is a sort of meta test - to test the functionality of "test_assert_path_exists"
+
+cimport cython
+
+@cython.test_assert_path_exists("//ReturnStatNode")
+def not_in_inner_compiler_directives():
+    # used to fail because ReturnStatNode wasn't in *this* CompilerDirectivesNode
+    with cython.boundscheck(False):
+        pass
+    return 1 # should pass
+
+@cython.test_assert_path_exists("//ReturnStatNode")
+def in_inner_compiler_directives():
+    # used to fail because ReturnStatNode wasn't in *this* CompilerDirectivesNode
+    with cython.boundscheck(False):
+        return 1
+
+# it's hard to come up with a corresponding test for fail_if_path_exists..