[libzstd] Fix ZSTD_decompressDCtx() with a dictionary

* `ZSTD_decompressDCtx()` did not use the dictionary loaded by
  `ZSTD_DCtx_loadDictionary()`.
* Add a unit test.
* A stacked diff uses `ZSTD_decompressDCtx()` in the
  `dictionary_round_trip` and `dictionary_decompress` fuzzers.

lib/decompress/zstd_decompress.c

@@ -788,7 +788,7 @@ size_t ZSTD_decompress_usingDict(ZSTD_DCtx* dctx,
 
 size_t ZSTD_decompressDCtx(ZSTD_DCtx* dctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize)
 {
-    return ZSTD_decompress_usingDict(dctx, dst, dstCapacity, src, srcSize, NULL, 0);
+    return ZSTD_decompress_usingDDict(dctx, dst, dstCapacity, src, srcSize, dctx->ddict);
 }