remove testoutputs from repository / add to .gitignore

Some unittests creates data which is stored within the repository, e.g.

`hubdc/test/test_core.py` creates data within a new folder `hubdc/test/output_core`.

To avoid commits of such temporary data it should be either written to a local directory that is not part of the repo or to a folder that is ignored by using the .gitignore file.

By now, the .gitignore ignores e.g. the following folder:

tmp/
doc/build/
*/build/
enmapboxtestdata/
test-reports/

We could introduce a test-outputs folder specifically for test results.

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