gnulib-tool.py: Avoid adding specific modules to a testdir.
Reported by Paul Eggert in
<https://lists.gnu.org/archive/html/bug-gnulib/2024-07/msg00292.html>.
* pygnulib/GLInfo.py (GLInfo.usage): Document
--extract-usability-in-testdir option.
* pygnulib/GLModuleSystem.py (GLModule.section_label_pattern): Recognize
Usable-in-testdir section.
(GLModule.getUsabilityInTestdir): New function.
* pygnulib/GLTestDir.py (GLTestDir.execute): Invoke it to filter out
modules to avoid. Warn if such a module was specified.
* pygnulib/main.py (main): Add support for
--extract-usability-in-testdir. Provide an error message for error 24.
* pygnulib/GLError.py: Likewise.
* modules/config-h (Usable-in-testdir): New section.
* modules/lib-ignore (Usable-in-testdir): New section.
* modules/mountlist (Usable-in-testdir): New section.
* modules/non-recursive-gnulib-prefix-hack (Usable-in-testdir): New
section.
* modules/timevar (Usable-in-testdir): New section.
(Files): Remove lib/timevar.def.