Specialize one for the SizedArray test helper (#58209)

Since the size of the array is encoded in the type, we may define `one`
on the type. This is useful in certain linear algebra contexts.

(cherry picked from commit d9fafab)

Author: jishnub

test/abstractarray.jl

@@ -1907,3 +1907,4 @@ end
         @test r2[i] == z[j]
     end
 end
+

test/testhelpers/SizedArrays.jl

@@ -29,6 +29,12 @@ Base.size(a::SizedArray) = size(typeof(a))
 Base.size(::Type{<:SizedArray{SZ}}) where {SZ} = SZ
 Base.getindex(A::SizedArray, i...) = getindex(A.data, i...)
 Base.zero(::Type{T}) where T <: SizedArray = SizedArray{size(T)}(zeros(eltype(T), size(T)))
+function Base.one(::Type{SizedMatrix{SZ,T,A}}) where {SZ,T,A}
+    allequal(SZ) || throw(DimensionMismatch("multiplicative identity defined only for square matrices"))
+    D = diagm(fill(one(T), SZ[1]))
+    SizedArray{SZ}(convert(A, D))
+end
+Base.parent(S::SizedArray) = S.data
 +(S1::SizedArray{SZ}, S2::SizedArray{SZ}) where {SZ} = SizedArray{SZ}(S1.data + S2.data)
 ==(S1::SizedArray{SZ}, S2::SizedArray{SZ}) where {SZ} = S1.data == S2.data
 function *(S1::SizedArray, S2::SizedArray)