pw_perf_test: Initialize the timer in the unit test

Change-Id: I10f1c82dec007b1351ea7200c86d2d468dc654f7
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/152174
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Taylor Cramer <cramertj@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>

pw_perf_test/timer_test.cc

@@ -20,12 +20,16 @@ namespace pw::perf_test::internal {
 namespace {
 
 TEST(TimerTest, DurationIsPositive) {
+  ASSERT_TRUE(TimerPrepare());
+
   Timestamp start = GetCurrentTimestamp();
   for (volatile int i = 0; i < 1000; i = i + 1) {
   }
   Timestamp end = GetCurrentTimestamp();
   int64_t duration = GetDuration(start, end);
   EXPECT_GT(duration, 0);
+
+  TimerCleanup();
 }
 
 }  // namespace