Fix P2PDownloadActionTest.

DownloadAction is now always using MultiRangeHttpFetcher, so had to
implement MultiRangeHttpFetcher::SetOffset() for unittest.

Bug: 73949420
Test: tests pass

Change-Id: I4ced2431e6a649402a318d601c384e998ef96b47
diff --git a/common/multi_range_http_fetcher.h b/common/multi_range_http_fetcher.h
index 8a91ead..54ddfbc 100644
--- a/common/multi_range_http_fetcher.h
+++ b/common/multi_range_http_fetcher.h
@@ -67,7 +67,7 @@
   }
 
   // HttpFetcher overrides.
-  void SetOffset(off_t offset) override {}  // for now, doesn't support this
+  void SetOffset(off_t offset) override;
 
   void SetLength(size_t length) override {}  // unsupported
   void UnsetLength() override {}