restructured test suite [WIP]

[chmerdon]

• tidied up test code
• added tests for Example210 and Example290
• new Example220 for heat equation on mixed geometries

(with the help of Qwen3.8)

Noticed: UnicodePlots on mixed geometries grid looks bad (this is fixed by WIAS-PDELib/ExtendableGrids.jl#145)

[chmerdon]

I don't know why the DownStream Test fails, I cannot reproduce this locally, but I triggered it also in #81 with a re-run of the Downstream Tester, so it it seems to be not related to this PR and maybe related to julia1.13 ?

[pjaap]

The downstream problem is caused by a circular dependency. Interesting how this ever worked.
ExFEM depends on ExFEMBase, and we add then ExFEM as a dependency to ExFEMBase in the downstream test.

Copilot suggests that a develop call should fix this, as an independent environment is created.

- run: |
    julia --project -e '
      using Pkg
      Pkg.develop(path=".")
      Pkg.add(url="https://github.com/WIAS-PDELib/ExtendableFEM.jl", rev="master")
      Pkg.instantiate()
      Pkg.test("ExtendableFEM")'

I'll add it to this PR.