Bug 40671

This patch fixes a problem I had using the IncludeController in version 2.2,
where nothing other then User Defined Variable assignments were being imported
from the included .jmx file.

Because everything needs to be placed under a TestPlan in the IncludeController,
before JMeter run, it traverses the subtree of the IncludeController; it only
adds nodes which are instanceOf Sampler or Controller. Because TestPlan is not
in these categories, JMeter just omits it and its children and that is why the
IncludeController does not work.

The solution is to only return the subtree under the TestPlan when JMeter asks
for the replacement subtree. However, it means that all user defined variables
defined in the TestPlan will be lost, so we need to make a copy for all the user
defined variables from the TestPlan. Because the user defined variable
controller works anywhere in the tree, so it just needs to copy them to
somewhere in the tree (I choose to copy them as a child under the TestPlan).

There is one problem though, the nested IncludeController will not work. I tried
to fix it but it seemed to be quite a big task. As the most important part is
working so I don’t want to spend too much time on it unless someone proves to me
that it is worthwhile fixing.