[#2149] Clean filesystem unit tests.

Problem
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there is a big of disorder in fs unit test

why we got this
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these are very old test and were the first test written for compat ... since then we got more experience


Changes
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For filesystem and setup spell checking I think that you should add those
works to your dictionary... there is no point in adding '-' just to make
a stupid spell checker happy.

We should not let our tools command us  .. but just help :)

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The new testing.FileSystemTestCase has the same name as a tests.normal.test_filesystem.FileSystemTest case


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I have removed the impersonated unit tests since they were just mocking and I prefer to do non mocked test in elevated.

For fs unit test with default avatar, a class instance is used since the FS instance is
the same for the same avatar.


How to test
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reviewers: @alibotean 

check that changes are ok.

Note that many tests were just moved ... and not rewritten... this is why there might be bad practices in them.

Thanks!