transport: Ensure stream context is cancelled in test (#8647)

transport: Ensure stream context is cancelled in test (#8647)

Fixes: #8646
The server stream's timer to monitor the deadline is closed when the
stream's cancel method is invoked.

https://github.com/grpc/grpc-go/blob/2d922719c02bb46f34482d592c35e72dc4a9ad92/internal/transport/http2_server.go#L623-L637

The cancel method is called when `closeStream` is called, just before it
calls `deleteStream`.

https://github.com/grpc/grpc-go/blob/2d922719c02bb46f34482d592c35e72dc4a9ad92/internal/transport/http2_server.go#L1347-L1357

The cancel method is not called in
[`deleteStream`](https://github.com/grpc/grpc-go/blob/2d922719c02bb46f34482d592c35e72dc4a9ad92/internal/transport/http2_server.go#L1302).

This change invokes `deleteStream` through `closeStream` in the flaking
test to ensure the stream is always cancelled to avoid leaking timers.

RELEASE NOTES: N/A