net: Use mockable time for tx download

src/random.cpp

@@ -667,6 +667,11 @@ uint64_t GetRand(uint64_t nMax) noexcept
     return FastRandomContext(g_mock_deterministic_tests).randrange(nMax);
 }
 
+std::chrono::microseconds GetRandMicros(std::chrono::microseconds duration_max) noexcept
+{
+    return std::chrono::microseconds{GetRand(duration_max.count())};
+}
+
 int GetRandInt(int nMax) noexcept
 {
     return GetRand(nMax);