Parallel Tests should not show up for serial builds

Parallel Tests should not show up for serial builds. There is no need to take an extra step to get a machine and collect TAP files.

https://ci.adoptium.net/job/Test_openjdk17_hs_extended.perf_ppc64le_linux/

Start with the linked CI job, Test_openjdk17_hs_extended.perf_ppc64le_linux, and inspect how its serial-build results present Parallel Tests. Trace the configuration or entry point that requests a machine and collects TAP files. Done means serial builds no longer display Parallel Tests or trigger that extra work, while parallel builds retain it.

Written by the indexing model from the issue text.