test_stopwatch: fix on mingw

There are some timing shenanigans with GCC's chrono that make this
unreliable. Add a start/stop and test for that to work around.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

tests/test_stopwatch.cpp

@@ -5,31 +5,40 @@
 TEST_CASE("stopwatch1", "[stopwatch]")
 {
     using std::chrono::milliseconds;
+    using clock = std::chrono::steady_clock;
     milliseconds wait_ms(250);
     milliseconds tolerance_ms(250);
-
+    auto start = clock::now();
     spdlog::stopwatch sw;
     std::this_thread::sleep_for(wait_ms);
-    REQUIRE(sw.elapsed() >= wait_ms);
-    REQUIRE(sw.elapsed() <= wait_ms + tolerance_ms);
+    auto stop = clock::now();
+    auto diff_ms = std::chrono::duration_cast<milliseconds>(stop - start);
+    REQUIRE(sw.elapsed() >= diff_ms);
+    REQUIRE(sw.elapsed() <= diff_ms + tolerance_ms);
 }
 
 TEST_CASE("stopwatch2", "[stopwatch]")
 {
     using spdlog::sinks::test_sink_st;
+    using std::chrono::duration_cast;
+    using std::chrono::milliseconds;
+    using clock = std::chrono::steady_clock;
 
-    std::chrono::duration<double> wait_duration(0.250);
-    std::chrono::duration<double> tolerance_duration(0.250);
+    clock::duration wait_duration(milliseconds(250));
+    clock::duration tolerance_duration(milliseconds(250));
 
     auto test_sink = std::make_shared<test_sink_st>();
 
+    auto start = clock::now();
     spdlog::stopwatch sw;
     spdlog::logger logger("test-stopwatch", test_sink);
     logger.set_pattern("%v");
     std::this_thread::sleep_for(wait_duration);
+    auto stop = clock::now();
     logger.info("{}", sw);
     auto val = std::stod(test_sink->lines()[0]);
+    auto diff_duration = duration_cast<std::chrono::duration<double>>(stop - start);
 
-    REQUIRE(val >= wait_duration.count());
-    REQUIRE(val <= (wait_duration + tolerance_duration).count());
+    REQUIRE(val >= diff_duration.count());
+    REQUIRE(val <= (diff_duration + tolerance_duration).count());
 }