Make tutorial filenames unique to avoid overwrites

[ellert]

This avoids problems during ctest like:

512/838 Test #516: tutorial-multicore-mp001_fillHistos .................................***Failed  Error regular expression found in output. Regex=[Error in <]  2.61 sec
Processing /builddir/build/BUILD/root-6.14.06/tutorials/multicore/mp001_fillHistos.C...
SysError in <TFile::TFile>: could not delete myFile_3.root (errno: 2) (No such file or directory)
Error in <TROOT::WriteTObject>: The current directory (Rint) is not associated with a file. The object (myHisto_3) has not been written.
(int) 0

[phsft-bot]

Can one of the admins verify this patch?

[dpiparo]

@ellert thanks a lot for this PR.