explicitly declare single threaded benchmark

[lplewa]

This causes that information about threads count is appended to benchmark name which makes it consistent with other bechmarks.

Description

Checklist

• Code compiles without errors locally
• All tests pass locally
• CI workflows execute properly

• CI workflows, not executed per PR (e.g. Nightly), execute properly
• New tests added, especially if they will fail without my changes
• Added/extended example(s) to cover this functionality
• Extended the README/documentation
• All newly added source files have a license
• All newly added source files are referenced in CMake files
• Logger (with debug/info/... messages) is used
• All API changes are reflected in docs and def/map files, and are tested