add error test for 'await' in generator expressions

tests/errors/pep492_badsyntax_async10.pyx

+# mode: error
+# tag: pep492, async
+
+async def genexpr(it):
+    return (await x for x in it)
+
+
+async def listcomp(it):
+    return [await x for x in it]
+
+
+async def setcomp(it):
+    return {await x for x in it}
+
+
+async def dictcomp(it):
+    return {await x:x+1 for x in it}
+
+
+# NOTE: CPython doesn't allow comprehensions either
+
+
+_ERRORS = """
+5:12: 'await' not allowed in generators (use 'yield')
+5:12: 'await' not supported here
+"""