[roottest] fix leak when using TSystem::ConcatFileName

[linev]

Return value must be released.

[ferdymercury]

Thanks for noticing! I think there are two more leaks with the same function in RooWorkspace.cxx ? Since the returned value is "copied" to a string rather than "moved" ? fyi @guitargeek
Or it should use the helper function TSystem.cxx:AssignAndDelete

[github-actions]

Test Results

    18 files      18 suites   3d 10h 25m 57s ⏱️
 2 741 tests  2 741 ✅ 0 💤 0 ❌
48 184 runs  48 184 ✅ 0 💤 0 ❌

Results for commit 48dca1e.

♻️ This comment has been updated with latest results.

[pcanal]

Thanks.