svn tests: use svn API to dispose of the test repositories.
svn tests: use svn API to dispose of the test repositories.

`repos.svn_repos_delete()` proves to be more robust than the manual `rmtree` version we had (see #8278).


git-svn-id: f68c6b3b1dcd5d00a2560c384475aaef3bc99487@10794 af82e41b-90c4-0310-8c96-b1721e28e2e2

moreati/trac-gitsvn,moreati/trac-gitsvn,moreati/trac-gitsvn,moreati/trac-gitsvn