Doc: make "Testing persistence-setup" clearer
While working on !81 (merged) I found the documentation on https://tails.boum.org/contribute/release_process/persistence-setup/ to be not as frictionless as I have liked it to be.
- I missed the part where I needed to change my directory
- I found the
export TAILS_GIT_CHECKOUT=part error prone.
So I propose this changes, in which the two code blocks can just be copy-pasted. I think that's easier. I also change the wording a bit.
A final note is on the last commit: I found many tests failing because of uninstalled dependencies (running debian buster), so I manually found which packages solved the problem and documented that in the Install test dependencies code block.