PIPE-3061: add pixi tasks for regression/unit tests and developer docs
Add two pixi tasks to pyproject.toml:
- test-regression: runs tests/regression/fast/ with pytest-xdist
(worksteal, -m "not mpi") inside a plain Python session
- test-pltest1: runs a single small ALMA-IF regression test via `casashell`
Both tasks cd into PL_WORKDIR (default: ../working) before launching,
keeping CASA logs and pipeline output out of the source tree.
PL_WORKDIR is overridable at invocation time:
PL_WORKDIR=/zfs/scratch/myrun pixi run test-regression
Also add docs/source/develdocmd/pixi_tasks.md documenting all five
pixi tasks (test-unit, test-regression, test-pltest1,
fetch-casarundata, build-docs), the available CASA environments,
PL_WORKDIR usage, and per-environment invocation examples.
