refactor: Use TranslateFn type consistently

The type was introduced in the previous commit.

src/test/util/setup_common.cpp

@@ -72,7 +72,7 @@ using node::LoadChainstate;
 using node::RegenerateCommitments;
 using node::VerifyLoadedChainstate;
 
-const std::function<std::string(const char*)> G_TRANSLATION_FUN = nullptr;
+const TranslateFn G_TRANSLATION_FUN{nullptr};
 
 constexpr inline auto TEST_DIR_PATH_ELEMENT{"test_common bitcoin"}; // Includes a space to catch possible path escape issues.
 /** Random context to get unique temp data dirs. Separate from m_rng, which can be seeded from a const env var */