Enable parallel TestPlan creation in qmake module

The testplan is created in qmake module (in modules.py) with option -j1 to avoid multiple processes corrupting the file. This should be relatively easy to fix by:

1. Write testplan to multiple files (TestPlan/1, TestPlan/2...) instead of single file (one can use test file name to create the target file)
2. Concatenate the multiple files into single file (TestPlan)

This would potentially save up to minutes of time for every build time.