Fix test element name changing inside loop causing a bug

[alimans3]

Inside a loop, one can use multiple variables in the label that could cause the name to change in different iterations.
For example, you can use ${__jm__Loop__idx} to add the iteration number to the label.
In this case, the parallel sampler fails to run any iteration other than the first due to the thread naming,
A unit test was added that would fail without the change

[alimans3]

@undera

[undera]

I don't work for Blazemeter anymore, sorry

[alimans3]

@sebastianlorenzo88 @dnyaneshnagre