Revert "tests: Use lowres_clock in time_it()"

This reverts commit e605a0368a.

lowres_clock is not updated when reactor is not running and this
variant of time_it() is not meant to be run in a rector.

tests/perf/perf.hh

@@ -19,10 +19,10 @@ void time_it(Func func, int iterations = 5) {
 
     for (int i = 0; i < iterations; i++) {
         auto start = clk::now();
-        auto end_at = lowres_clock::now() + std::chrono::seconds(1);
+        auto end_at = start + std::chrono::seconds(1);
         uint64_t count = 0;
 
-        while (lowres_clock::now() < end_at) {
+        while (clk::now() < end_at) {
             for (int i = 0; i < 10000; i++) { // amortize clock reading cost
                 func();
                 count++;