Resolve "[CI] Wrong test vectors used in external split rendering "compare" tests"

• Related issues: #1417 (closed)
• Requested reviewers:

Reason why this change is needed

In split rendering tests, a different set of input files is selected, depending on whether the test compares REF and CUT, or runs "standalone".

For external split rendering tests this logic is broken and short smoke test vectors are always used.

Description of the change

Logic from full-chain split rendering is also used for external split rendering

Affected operating points

N/A

Related to #1417 (closed)