执行顺序：
0 configuration elements
1 pre-processor elements
2 timers
3 samplers
4 post processors(unless sampleresult is null)
5 assertions (unless sampleresult is null)
6 listeners (unless sampleresult is null)
注意：timer pre/post processors assertions 有应用的样本器时才被处理；samplers/logic controllers 按在树中的顺序执行，其他元素被找到的范围和类型处理
例如：controller:
post-processor1
sampler1
sampler2
timer1
assertion1
pre-processor1
timer2
post-processor2
执行顺序：
pre-processor1
timer1
timer2
sampler1
post-processor1
post-processor2
assertion1
pre-processor1
timer1
timer2
sampler2
post-processor1
post-processor2
assertion1