Fix PythonTest_solenv_python for Linux --enable-mergelibs=more

Fix PythonTest_solenv_python for Linux --enable-mergelibs=more

...where it recently started to fail for me with

> ======================================================================
> ERROR: test_gbuildtojson (gbuildtojson.CheckGbuildToJsonModules)
> ----------------------------------------------------------------------
> Traceback (most recent call last):
>   File "solenv/qa/python/gbuildtojson.py", line 147, in test_gbuildtojson
>     jsonfiles = os.listdir(os.path.join(self.tempwork, 'GbuildToJson', 'Library'))
> FileNotFoundError: [Errno 2] No such file or directory: '/tmp/gbuildffqxqxbf/GbuildToJson/Library'

presumably because none of the modules iterated over there happen to put any
files into the GbuildToJson/Library directory (and, by doing so, would create
that directory)

Change-Id: I2622f300d6d9c45d28a98fb6bb82aa684d1d5507
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/165033
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>