Tweak diagnostic message based on my understanding of the code

Signed-off-by: Jesse Vincent <jesse@keyboard.io>

tests/simulator/timestamps/test/testcase.cpp

@@ -40,7 +40,7 @@ TEST_F(ReportTimestamps, Keyboard) {
 
   ASSERT_EQ(state->HIDReports()->Keyboard().size(), 1);
   EXPECT_THAT(state->HIDReports()->Keyboard(0).Timestamp(), current_time)
-      << "The report should have a correct timestamp";
+      << "Report timestamp is the same as the simulator clock as of the end of the last cycle";
 
   sim_.RunForMillis(delay);
   current_time += delay;
@@ -51,7 +51,7 @@ TEST_F(ReportTimestamps, Keyboard) {
 
   ASSERT_EQ(state->HIDReports()->Keyboard().size(), 1);
   EXPECT_THAT(state->HIDReports()->Keyboard(0).Timestamp(), current_time)
-      << "The report should have a correct timestamp";
+      << "Report timestamp is the same as the simulator clock as of the end of the last cycle";
 }
 
 }  // namespace