libstore: update exceptionAbortsDownload test

the exception no longer actually matters, only that the download stream
is destroyed before the download finishes does. exceptions during drain
calls on the returned stream will cause the stream to be destroyed, but
draining this stream is just a remnant of the old download-to-sink API.

Change-Id: Ic6de40deb2ccff09d77180148afe746f29f55d23

tests/unit/libstore/filetransfer.cc

@@ -203,23 +203,16 @@ serveHTTP(std::string status, std::string headers, std::function<std::string()>
     return serveHTTP({{{status, headers, content}}});
 }
 
-TEST(FileTransfer, exceptionAbortsDownload)
+TEST(FileTransfer, destructionAbortsDownload)
 {
-    struct Done : BaseException
-    {};
-
     AsyncIoRoot aio;
     auto ft = makeFileTransfer();
 
-    LambdaSink broken([](auto block) { throw Done(); });
-
     auto [port, srv] = serveHTTP({{"200 ok", "", [](int) { return "foo"; }}});
-    ASSERT_THROW(
-        aio.blockOn(
-            aio.blockOn(ft->download(fmt("http://[::1]:%d/index", port))).second->drainInto(broken)
-        ),
-        Done
-    );
+    // discard the download stream. this must cancel the download, even when the
+    // remote still has data to send. we simulate this by sending the same block
+    // of data over and over without any content-length headers sent the client.
+    (void) aio.blockOn(ft->download(fmt("http://[::1]:%d/index", port)));
 
     // makeFileTransfer returns a ref<>, which cannot be cleared. since we also
     // can't default-construct it we'll have to overwrite it instead, but we'll