Testsuite driver: replace "extra_clean" by "git clean -X"

When adding a new test that creates extra files, one also has to do some bookkeeping:

1. Supply an extra_clean argument to the test function in all.T.
2. Add an entry in testsuite/.gitignore

Step number 1 is often forgotten. Currently there are over 200 uncleaned files and directories left behind in the testsuite, after running validate and then make clean (checked with git clean -X -d -n | wc -l).

I propose to replace the functionality that extra_clean provides, namely to remove these extra files when one calls make clean, by doing the equivalent of git clean -X -d:

    -X 
        Remove only files ignored by Git.

    -d 
        Remove untracked directories in addition to untracked files. ...

That is: use the information from the .ignore files to figure out which files to delete on make clean.

The advantages of making this change are:

1. Less manual bookkeeping, less work, less code
2. A more thorough make clean

Implementation detail: we can not just call git clean directly, since this should also work when we are not in a git directory (for example a build directory created with lndir).

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