[WIP] Fix _copy_decouple() for relative symlinks

[abadger]

It was found in testing that absolute symlinks in /etc/pki are being preserved correctly but relative symlinks are causing a traceback. This PR will fix that.

Right now this is just additional unittests for the desired behaviour:

• Add some unittests that test symlink handling of copy_decouple with relative symlinks.
• Enhance the temporary_directory fixture to handle creation of relative symlinks too.

If anyone has additional scenarios that they think should work feel free to add to the PR or mention them to me and I'll make a test for it.

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[abadger]

I pushed the branch to oamg/leapp-repository by mistake (instead of abadger/leapp-repository). Closing this in favor of: https://github.com/oamg/leapp-repository/pull/1160