lDebug tests uncovered a bug

Today, running the lDebug tests turned up a recent bug, now fixed. With this change all tests pass for build_name=debug, build_name=debugx, as well as build_name=debug test_booting=1.

The offending test was for checking part of the error message for writing to a read-only variable. I have largely neglected the test suite for a while now but perhaps I will expand it again soon.