[test] fix name conflict when multithreading

[ferdymercury]

This Pull request:

Changes or fixes:

see https://github.com/root-project/root/actions/runs/17157474273/job/48677881584?pr=19688

since TestObj.cpp creates same-named file in same directory.

[linev]

Do you have example of such conflict?

File "test.root" created in dedicated directory for root/tree/array tests
and therefore should not conflict with any other tests

[ferdymercury]

Do you have example of such conflict?

https://github.com/root-project/root/actions/runs/17157474273/job/48677881584?pr=19688

File "test.root" created in dedicated directory for root/tree/array tests

TestObj.cpp creates same-named file in the same root/tree/array directory.

[linev]

Now I see. Thanks to fixing it.

[github-actions]

Test Results

    21 files      21 suites   3d 11h 42m 30s ⏱️
 3 561 tests  3 424 ✅  0 💤 137 ❌
72 974 runs  72 820 ✅ 17 💤 137 ❌

For more details on these failures, see this check.

Results for commit a4c3cc4.