test: move the matrix-function tests into the TestSuite

The four matrix functions were tested by hand-written, CPU-only top-level
files plus per-backend duplicates under `test/genericschur/` and
`test/genericlinearalgebra/`, while every decomposition lives in
`test/testsuite/` as array-type-parameterized functions driven by a thin
`test/<area>/<name>.jl`. Close that gap: `test/testsuite/matrixfunctions/`
now holds the bodies and `test/matrixfunctions/` the drivers.

Assertions become invariants rather than comparisons against
`LinearAlgebra`, so one body serves host, CUDA, ROCm and downstream array
types: `sqrtA * sqrtA ≈ A`, `exponential(logA) ≈ A`, `exp(A) * exp(-A) ≈ I`,
and elementwise spectrum mapping via `eigh_vals`. Reference cross-checks
against `LinearAlgebra` are kept, but confined to `test_*_reference`, which
the drivers only call for host arrays.

GPU coverage now falls out of the array-type parameterization, so the
bespoke `test_*_gpu` functions are gone. Three new instantiators
(`instantiate_offaxis_matrix`, `instantiate_posdef_matrix`,
`instantiate_hermitian_spectrum`) give the controlled spectra these
functions need, reusing `instantiate_unitary` and the `Diagonal` handling
already in the suite.

The domain rules differ per function and are now encoded rather than
approximated: a roundoff-negative eigenvalue clamps for `squareroot` and
positive fractional `power`, but still throws for `logarithm`, where
clamping onto zero leaves the matrix singular. `MatrixFunctionViaLA`
inspects only the realness of its result, so it accepts singular input that
the spectrum-based algorithms reject; the `eigh`-based algorithms reject a
negative eigenvalue whatever the scalar type, since the root of a hermitian
matrix with a negative eigenvalue is not hermitian.

Also drops the deprecated `LAPACK_Simple`/`LAPACK_QRIteration`/
`GS_QRIteration`/`GLA_QRIteration` spellings, which accounted for the
deprecation warnings these tests emitted. Generic eltypes name their driver
explicitly (`QRIteration(; driver = GS())`), since with both generic
packages loaded `default_driver` resolves `QRIteration` to GLA, which
provides no `geev!`.

Full suite: 93826 passing, up from 90917.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: lkdvos