test(boottime): kill vm on every iteration

If the VM is not killed, residual effects of VM execution
from the previous iteration affect boot time measurements
in the following iteration.

Signed-off-by: Nikita Kalyazin <[email protected]>

Author: kalyazin

tests/integration_tests/performance/test_boottime.py

@@ -210,3 +210,4 @@ def test_boottime(
             boottime_us - build_time.microseconds,
             unit="Microseconds",
         )
+        vm.kill()