fixup! component/nxdtest: Prepare for nexedi/nxdtest!13

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.

component/nxdtest/test.cfg

@@ -17,7 +17,7 @@ egg = nxdtest[test]
 [python-interpreter]
 eggs +=
   ${pytest:eggs}
-  ${nxdtest:eggs}
+  ${nxdtest-egg:egg}
 
 # env.sh for interpreter to be on $PATH.
 [nxdtest-env.sh]