generate.py: disallow generating test suites for multiple paths
generate.py: disallow generating test suites for multiple paths

Our generate.py script is used to extract and write test suite
declarations into the clar.suite file. As is, the script accepts
multiple directories on the command line and will generate this file for
each of these directories.

The generate.py script will always write the clar.suite file into the
directory which is about to be scanned. This actually breaks
out-of-tree builds with libgit2, as the file will be generated in the
source tree instead of in the build tree. This is noticed especially in
the case where the source tree is mounted read-only, rendering us unable
to build unit tests.

Due to us accepting multiple paths which are to be scanned, it is not
trivial to fix though. The first solution which comes into mind would be
to re-create the directory hierarchy at a given output path or use
unique names for the clar.suite files, but this is rather cumbersome and
magical. The second and cleaner solution would be to fold all
directories into a single clar.suite file, but this would probably break
some use-cases.

Seeing that we do not ever pass multiple directories to generate.py, we
will now simply retire support for this. This allows us to later on
introduce an additional option to specify the path where the clar.suite
file will be generated at, defaulting to "clar.suite" inside of the
scanned directory.

vmg/clar,vmg/clar