test(hex8): await every worker before first paired linear call

tests/unit/elements/hex8_high_low_test.cpp

@@ -3,6 +3,7 @@
 #include <gtest/gtest.h>
 
 #include <array>
+#include <atomic>
 #include <cfenv>
 #include <cmath>
 #include <cstdint>
@@ -10,6 +11,7 @@
 #include <future>
 #include <limits>
 #include <random>
+#include <thread>
 #include <utility>
 #include <vector>
 
@@ -146,13 +148,17 @@ TEST(Hex8HighLow, ConcurrentFirstLinearTrialsAcrossCopiesAndMoveKeepBits) {
   ASSERT_TRUE(expected_b.HasValue());
   std::promise<void> start;
   const auto signal = start.get_future().share();
+  std::atomic<std::size_t> arrived{0U};
   std::array<std::future<fesa::Result<fesa::Hex8HighLowResponse>>, 4> calls;
   for (std::size_t i = 0; i < calls.size(); ++i)
     calls[i] = std::async(std::launch::async, [&, i] {
+      arrived.fetch_add(1U, std::memory_order_release);
       signal.wait();
       return (i % 2U == 0U ? element : moved)
           .EvaluateLinearHighLow(high, i < 2U ? low_a : low_b);
     });
+  while (arrived.load(std::memory_order_acquire) != calls.size())
+    std::this_thread::yield();
   start.set_value();
   for (std::size_t i = 0; i < calls.size(); ++i) {
     const auto response = calls[i].get();