njplot test regression since 24.2.0-rc4 (rc3 was fine)

the third regression caused by 24.2.0 on Debian CI is seen with njplot

 71s autopkgtest [10:31:12]: test run-unit-test: [-----------------------
 72s 2580
 72s TEST START
 72s X server has no GLX extension - required to use OpenGL!
 72s autopkgtest [10:31:13]: test run-unit-test: -----------------------]
 72s run-unit-test        FAIL non-zero exit status 10

full log https://ci.debian.net/packages/n/njplot/unstable/amd64/50491876/

successful run with rc3: https://ci.debian.net/packages/n/njplot/unstable/amd64/50013482/

and the test: https://sources.debian.org/src/njplot/2.4-9/debian/tests/run-unit-test/?hl=8#L8