Add missing dependency to quiche_test_support

The build currently fails with the following error:

```
quiche/quic/test_tools/quic_connection_peer.cc:12:10: error: module //quiche:quiche_test_support does not depend on a module exporting 'absl/types/variant.h'
#include "absl/types/variant.h"
         ^
```

PiperOrigin-RevId: 696162022

quiche/BUILD.bazel

diff --git a/quiche/BUILD.bazel b/quiche/BUILD.bazel
index b6450ed..94dcf51 100644
--- a/quiche/BUILD.bazel
+++ b/quiche/BUILD.bazel
@@ -291,6 +291,7 @@
         "@com_google_absl//absl/strings:str_format",
         "@com_google_absl//absl/types:optional",
         "@com_google_absl//absl/types:span",
+        "@com_google_absl//absl/types:variant",
         "@com_google_googletest//:gtest",
     ],
 )