Fix the following build failure in nxdtest's own test:

    [2021-12-10 15:57:15,142] INFO     While:
    [2021-12-10 15:57:15,142] INFO       Installing.
    [2021-12-10 15:57:15,142] INFO       Getting section python-interpreter.
    [2021-12-10 15:57:15,142] INFO       Initializing section python-interpreter.
    [2021-12-10 15:57:15,142] INFO       Getting option python-interpreter:eggs.
    [2021-12-10 15:57:15,142] INFO       Getting section nxdtest.
    [2021-12-10 15:57:15,142] INFO       Initializing section nxdtest.
    [2021-12-10 15:57:15,142] INFO       Getting option nxdtest:eggs.
    [2021-12-10 15:57:15,142] INFO       Getting section .nxdtest.pyexe.
    [2021-12-10 15:57:15,142] INFO       Initializing section .nxdtest.pyexe.
    [2021-12-10 15:57:15,142] INFO       Getting option .nxdtest.pyexe:eggs.
    [2021-12-10 15:57:15,142] INFO       Getting option nxdtest:eggs.
    [2021-12-10 15:57:15,142] INFO       Getting option .nxdtest.pyexe:eggs.
    [2021-12-10 15:57:15,142] INFO     Error: Circular reference in substitutions.

This is hot, not a proper, fix to recover nxdtest.UnitTest-Master status.
I will think more calmly what to properly do.