Write missing tests for parity with ext_proc client interceptor tests.

Author: kannanjgithub

xds/src/main/java/io/grpc/xds/ExternalProcessorServerInterceptor.java

@@ -905,6 +905,7 @@ public void close(Status status, Metadata trailers) {
           if (markDataPlaneCallClosed()) {
             proceedWithClose(status, trailers);
           }
+          closeExtProcStream();
           return;
         }