Really support milliseconds in wxLog

Although this was supposed to work, specifying "%l" in wxLog time stamp
format actually didn't because wxLog timestamps were stored as seconds.

Fix this by storing them as milliseconds and add a simple test (not
executed by default) showing that "%l" works correctly now.

Still keep the old wxLogRecordInfo::timestamp field for compatibility,
as it was documented.

See #13059.

tests/log/logtest.cpp

@@ -454,4 +454,16 @@ void macroCompilabilityTest()
         wxLogDebug("hello debug %d", 42);
 }
 
+// This allows to check wxLogTrace() interactively by running this test with
+// WXTRACE=logtest.
+TEST_CASE("wxLog::Trace", "[log][.]")
+{
+    // Running this test without setting WXTRACE is useless.
+    REQUIRE( wxGetEnv("WXTRACE", NULL) );
+
+    wxLogTrace("logtest", "Starting test");
+    wxMilliSleep(250);
+    wxLogTrace("logtest", "Ending test 1/4s later");
+}
+
 #endif // wxUSE_LOG