message: Add a test case for Quic BufWriter
  Summary:
  forgot about this coverage in the original diff introduced the
  BufWriter
diff: @@ -487,3 +487,18 @@ TEST(BufWriterTest, BufQueueChainCopyTooLargeLimit) {
  reader.readFixedString(
  testStr1.size() + testStr2.size() + testStr3.size()));
  }
  +
  +TEST(BufWriterTest, TwoWriters) {
  + auto outputBuffer = folly::IOBuf::create(1000);
  + auto inputBuffer = folly::IOBuf::copyBuffer("Destroyer");
  + BufWriter bufWriter(*outputBuffer, 1000);
  + bufWriter.insert(inputBuffer.get());
  + EXPECT_EQ(9, outputBuffer->length());
  +
  + BufWriter bufWriter2(*outputBuffer, outputBuffer->length());
  + auto inputBuffer2 = folly::IOBuf::copyBuffer(" Saint");
  + bufWriter2.insert(inputBuffer2.get());
  + folly::io::Cursor reader(outputBuffer.get());
  + EXPECT_EQ(15, outputBuffer->length());
  + EXPECT_EQ("Destroyer Saint", reader.readFixedString(outputBuffer->length()));
  +}
  