Test cleanup

[filoferra]

Reorganises test/ without changing what the library does. Each commit is a step you can review on its own.

1. Tests the suite had stopped running. The four files testing the manual's examples are listed in runtests.jl again. Their stale skips are now real tests: IS set operations, and ASCII and binary viewer I/O. The Tao examples run in a fresh process. In PETSc 3.25 the LMVM-based Tao types fail after any earlier cycle that created a KSP, and docs/src/man/tao_lowlevel.md now says so. 01-hello.jl is removed, and the SNES helper test runs once instead of twice.

2. One folder per group. test/ is split into core, vecmat, solvers, dm, lowlevel, regression, examples and mpi, and the file-name prefixes are gone. Pkg.test("PETSc"; test_args = ["dm", "mpi"]) runs only the named groups, and an unknown name raises an error. Paths go through pkgdir(PETSc). scripts/generate_renames.jl now writes test/core/deprecations.jl, and docs/src/man/contributing.md has a "Running the tests" section.

3. Less duplication.

• The two DMStag files stay separate: dm/dmstag.jl runs on any number of ranks, and dm/dmstag_serial.jl checks exact values on one rank. Each file now says which it is at the top, and their testset names no longer clash.
• solvers/snes_callbacks.jl and the SNES half of core/smoke.jl solved the same 2×2 system and still taught the return 0 callback rule. That system is now tested once, in solvers/snes.jl, with callbacks that return nothing, in both the named-function and do-block forms.
• The @assert-only snes_helpers.jl and tao_helpers.jl become lowlevel/readers.jl, which uses @test.
• 19 files used getlib(PetscScalar = Float64), which assumed an Int64 build. They now take the loaded library from PETSc.petsclibs[1].