Revert "Remove test that breaks the build"

This reverts commit 7fe3be8d5f24db9837fff7e5b2fc45dcb464eef0.

tests/CMakeLists.txt

@@ -7,4 +7,9 @@ if(UNIX)
     if(NOT APPLE)
         target_link_libraries(${PROJECT_NAME}-test util)
     endif()
+
+    if(NOT APPLE)  # these tests are unreliable on macOS
+      ament_add_gtest(${PROJECT_NAME}-test-timer unit/unix_timer_tests.cc)
+      target_link_libraries(${PROJECT_NAME}-test-timer ${PROJECT_NAME})
+    endif()
 endif()

tests/unit/unix_timer_tests.cc

@@ -0,0 +1,63 @@
+#include "gtest/gtest.h"
+#include "serial/impl/unix.h"
+
+#include <unistd.h>
+#include <stdlib.h>
+
+using serial::MillisecondTimer;
+
+namespace {
+
+/**
+ * Do 100 trials of timing gaps between 0 and 19 milliseconds.
+ * Expect accuracy within one millisecond.
+ */
+TEST(timer_tests, short_intervals) {
+  for (int trial = 0; trial < 100; trial++)
+  {
+    uint32_t ms = rand() % 20;
+    MillisecondTimer mt(ms);
+    usleep(1000 * ms);
+    int32_t r = mt.remaining(); 
+
+    // 1ms slush, for the cost of calling usleep.
+    EXPECT_NEAR(r+1, 0, 1);
+  }
+}
+
+TEST(timer_tests, overlapping_long_intervals) {
+  MillisecondTimer* timers[10];
+
+  // Experimentally determined. Corresponds to the extra time taken by the loops,
+  // the big usleep, and the test infrastructure itself.
+  const int slush_factor = 14;
+
+  // Set up the timers to each time one second, 1ms apart.
+  for (int t = 0; t < 10; t++)
+  {
+    timers[t] = new MillisecondTimer(1000);
+    usleep(1000);
+  }
+
+  // Check in on them after 500ms.
+  usleep(500000);
+  for (int t = 0; t < 10; t++)
+  {
+    EXPECT_NEAR(timers[t]->remaining(), 500 - slush_factor + t, 5);
+  }
+
+  // Check in on them again after another 500ms and free them.
+  usleep(500000);
+  for (int t = 0; t < 10; t++)
+  {
+    EXPECT_NEAR(timers[t]->remaining(), -slush_factor + t, 5);
+    delete timers[t];
+  }
+}
+
+}  // namespace
+
+int main(int argc, char **argv) {
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}