CMake: build serial unit tests without MPI

[Stardust0831]

Linked Issue

No linked issue. This PR corrects the CMake conditions that prevent serial unit tests from being built without MPI.

Unit Tests and/or Case Tests for my changes

• Commands run: cmake -S . -B build -DBUILD_TESTING=ON -DENABLE_MPI=OFF; built the affected serial test targets; ran the focused CTest selection; git diff --check; repository governance check.
• Result summary: All eight affected serial tests passed with MPI disabled; one existing benchmark test was skipped by its own guard.
• Checks not run, with reason: MPI-dependent suites were not run because this change targets their existing ENABLE_MPI gating.

What's changed?

• Register source_io/test_serial and plane-wave test_serial whenever BUILD_TESTING is enabled, independent of MPI.
• Keep parallel, kernel, GPU, and mpirun-based suites behind ENABLE_MPI.
• Enter source_cell/test for serial builds so its existing test-only cell_info object is available, while returning before the MPI-only tests when MPI is disabled.
• Keep test source ownership in the existing test directories and leave production targets unchanged.

Governance Notes

• INPUT/docs changes: None.
• Core module impact: CMake test registration only; production code and production targets are unchanged.
• Exceptions requested: None.

[mohanchen]

LGTM