tests: Add tests for generator throw and yield-from with exc handlers.

This commit adds a generator test for throwing into a nested exception, and
one when using yield-from with a pending exception cleanup.  Both these
tests currently fail on the native emitter, and are simplified versions of
native test failures from uasyncio in #5332.

tests/basics/gen_yield_from_pending.py

@@ -0,0 +1,23 @@
+# Tests that the pending exception state is managed correctly
+# (previously failed on native emitter).
+
+def noop_task():
+    print('noop task')
+    yield 1
+
+def raise_task():
+    print('raise task')
+    yield 2
+    print('raising')
+    raise Exception
+
+def main():
+    try:
+        yield from raise_task()
+    except:
+        print('main exception')
+
+    yield from noop_task()
+
+for z in main():
+    print('outer iter', z)

tests/basics/generator_throw_nested.py

@@ -0,0 +1,33 @@
+# Tests that the correct nested exception handler is used when
+# throwing into a generator (previously failed on native emitter).
+
+def gen():
+    try:
+        yield 1
+        try:
+            yield 2
+            try:
+                yield 3
+            except Exception:
+                yield 4
+                print(0)
+            yield 5
+        except Exception:
+            yield 6
+            print(1)
+        yield 7
+    except Exception:
+        yield 8
+        print(2)
+    yield 9
+
+for i in range(1, 10):
+    g = gen()
+    try:
+        for _ in range(i):
+            print(next(g))
+        print(g.throw(ValueError))
+    except ValueError:
+        print('ValueError')
+    except StopIteration:
+        print('StopIteration')