Get all futures in test_concurrent_append_flush

In test_concurrent_append_flush, which is a fuzzer style test,
we now get() all futures returned by flush calls during the fuzz
portion, instead of only the last flush.

It is possible in some cases for prior futures to be unavailable
even after the last future has resolved which caused occasional
CI failures. See 13035 for more analysis.

Fixes redpanda-data#13035.

src/v/storage/tests/log_segment_appender_test.cc

@@ -455,6 +455,17 @@ static void run_concurrent_append_flush(
             last_append->get();
         }
 
+        // append a final flush, so we are in a known flushed state for the
+        // following checks
+        futs.emplace_back(appender.flush());
+
+        for (auto& f : futs) {
+            // get all the flush futures
+            // whp these are all available except possibly the last one
+            // (appended above) but this is not actually guaranteed, see
+            // redpanda#13035
+            f.get();
+        }
         auto sa_state = fmt::format("{}", appender);
 
         // now there should be nothing in-flight