Fix the failing warmup and duration are cumulative (#2854)

We need to verify total duration was at least 2 seconds, elapsed time
can be quite variable to check upper-bound

Fixes https://github.com/valkey-io/valkey/issues/2843

Signed-off-by: Roshan Khatri <rvkhatri@amazon.com>

tests/integration/valkey-benchmark.tcl

@@ -217,7 +217,7 @@ tags {"benchmark network external:skip logreqres:skip"} {
 
             # Verify total duration was at least 2 seconds
             set elapsed [expr {($end_time - $start_time)/1000.0}]
-            assert {$elapsed >= 2 && $elapsed <= 2.25}
+            assert {$elapsed >= 2}
 
             # Check reported duration
             lassign [regexp -inline {(\d+) requests completed in ([\d.]+) seconds} $output] -> requests duration