[lmi] --gui_test_path and implicitly created output files

[Vadim Zeitlin]

 Hello,

 It turns out that implementing --gui_test_path is not completely trivial
as some files are unconditionally created by lmi in the current directory:
this is the case for at least the files created by mec_server which is used
from mec_view with just base_filename() as the file name (i.e. "unnamed")
and ".monthly_trace" debugging files created when illustrations debugging
is enabled. In both cases we can't currently force creation of these files
in the directory specified by the --gui_test_path command line option.

 I see the following possible solutions:

0. Ignore the problem for these files and just ensure that they are deleted
   after the end of the test (currently this doesn't happen neither, as
   they are created as a side effect and not explicitly, so I didn't even
   know originally that they were created by lmi).

1. Solve the problem by changing the current directory before executing the
   code creating these files and restoring it after doing it. I didn't test
   this yet, but I think it should work. I also however dislike this
   solution because it's fragile and could result in subtle (i.e. the worst
   sort) differences between the code behaviour when it's being tested and
   when it's being used normally.

2. Change lmi itself to allow customizing the directory used for these
   files creation. I don't know if you would agree to doing this, but I'm
   afraid this is unavoidable if we want to really solve this problem, i.e.
   if (0) is not good enough.

 Please let me know if you see any other possibilities or if I should go
ahead with (2) and propose something concrete.

 Thanks in advance,
VZ