Improvement: Runtime of nca test

[Davknapp]

Describe your changes here:
Closes #1647
changed a little the recursion depth.
The most time was spend on the last test in the testsuite, because it always tested up to maxlvl. Reduced it down to maxlvl/2 for test levels other than 0.

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[ole-alb]

As you changed the name of ´recursion_depthin tostart_level` in the last test, can you please also change it in the comment in line 259? I wasn't able to make suggested change.