Simplify and correct some aspects of the test
Simplify and correct some aspects of the test

- Change the directory into the TBufferMergerFiles
- Do not store in a unique_ptr a TTree extracted from a TFile
- Use normal files and not unique ptr to them: we know they reside on disk, no need to use the plugin mngr

bbockelm/root,zzxuanyuan/root,agarciamontoro/root,mhuwiler/rootauto,BerserkerTroll/root,agarciamontoro/root,BerserkerTroll/root,karies/root,karies/root,mhuwiler/rootauto,karies/root,root-mirror/root,agarciamontoro/root,root-mirror/root,simonpf/root,davidlt/root,mhuwiler/rootauto,davidlt/root,davidlt/root,olifre/root,zzxuanyuan/root,root-mirror/root,olifre/root,simonpf/root,zzxuanyuan/root,davidlt/root,bbockelm/root,mhuwiler/rootauto,mhuwiler/rootauto,davidlt/root,karies/root,root-mirror/root,zzxuanyuan/root,root-mirror/root,bbockelm/root,agarciamontoro/root,agarciamontoro/root,karies/root,davidlt/root,root-mirror/root,bbockelm/root,BerserkerTroll/root,BerserkerTroll/root,olifre/root,BerserkerTroll/root,root-mirror/root,olifre/root,agarciamontoro/root,zzxuanyuan/root,root-mirror/root,zzxuanyuan/root,bbockelm/root,root-mirror/root,zzxuanyuan/root,BerserkerTroll/root,simonpf/root,mhuwiler/rootauto,simonpf/root,simonpf/root,davidlt/root,root-mirror/root,zzxuanyuan/root,simonpf/root,root-mirror/root,bbockelm/root,BerserkerTroll/root,mhuwiler/rootauto,karies/root,bbockelm/root,olifre/root,olifre/root,davidlt/root,zzxuanyuan/root,karies/root,karies/root,bbockelm/root,simonpf/root,mhuwiler/rootauto,bbockelm/root,olifre/root,zzxuanyuan/root,bbockelm/root,olifre/root,BerserkerTroll/root,davidlt/root,agarciamontoro/root,mhuwiler/rootauto,BerserkerTroll/root,agarciamontoro/root,olifre/root,simonpf/root,olifre/root,simonpf/root,BerserkerTroll/root,olifre/root,agarciamontoro/root,agarciamontoro/root,karies/root,simonpf/root,BerserkerTroll/root,agarciamontoro/root,zzxuanyuan/root,bbockelm/root,simonpf/root,karies/root,karies/root,mhuwiler/rootauto,mhuwiler/rootauto,davidlt/root,zzxuanyuan/root,davidlt/root