test: refactor existing appendable upload tests to work with new implementation

* Add incremental checksums to all messages
* Add final object_checksums for all finalization
* Remove tests that were testing the same thing -- especially when the setup was fairly involved
* Remove some Integration tests that are now covered by unit tests of the bidi stream / bidi upload state (redirect limits, etc)
* Refactor lots of common messages and test run flows to reduce duplication and make debugging easier
* Ignore some tests that previously would partially split data rather than messages -- the current implementation only evicts when a whole message is consumed
* In general, signal `onComplete()` after `onNext` in response to a finalize message
* Update ITAppendableUploadTest to run for multiple permutations of BlobAppendableUploadConfigs

Author: BenWhitehead

google-cloud-storage/src/main/java/com/google/cloud/storage/Buffers.java

@@ -173,6 +173,21 @@ static int fillFrom(ByteBuffer buf, ReadableByteChannel c) throws IOException {
     return total;
   }
 
+  static int emptyTo(ByteBuffer buf, WritableByteChannel c) throws IOException {
+    int total = 0;
+    while (buf.hasRemaining()) {
+      int written = c.write(buf);
+      if (written != -1) {
+        total += written;
+      } else if (total == 0) {
+        return -1;
+      } else {
+        break;
+      }
+    }
+    return total;
+  }
+
   static long totalRemaining(ByteBuffer[] buffers, int offset, int length) {
     long totalRemaning = 0;
     for (int i = offset; i < length; i++) {