Improve ParU test structure when LTS upgrades to Julia 1.12

[AJ0070]

Currently ParUFactorization tests are integrated into the Core test group behind a
VERSION >= v"1.12.0-" guard, because ParU_jll cannot be added to [extras]/[targets]
on Julia 1.10 (current LTS) - it requires SuiteSparse_jll ≥ 7.8 which is only bundled
with Julia 1.12+.

Once the LTS advances to Julia 1.12, we should:

• Add ParU_jll back to [extras] and [targets] in Project.toml
• Move the ParU tests out of the VERSION >= v"1.12.0-" guard in test/runtests.jl
• Remove test/paru/ sub-environment (or keep it — up for discussion)

Tracked from: #895