System.Numerics.Tensors.Net8.Tests does not appear to actually run against .NET 8 #101846
Description
As per the title, the System.Numerics.Tensors.Net8.Tests do not appear to currently be running against .NET 8 as intended. Instead, they are compiled for .NET 8 and appear to be running on the .NET 9 runtime via the roll forward mechanism.
This can be seen in one of the CI legs for #101800 where the Net8.Tests project has the following failure:
System.Numerics.Tensors.Tests.SingleGenericTensorPrimitives.SpanDestinationFunctions_SpecialValues(tensorPrimitivesMethod: SpanDestinationDelegate { Method = Void ReciprocalSqrtEstimate[Single](System.ReadOnlySpan`1[System.Single], System.Span`1[System.Single]), Target = null }, expectedMethod: Func`2 { Method = Single ReciprocalSqrtEstimate(Single), Target = null }, tolerance: 0.01171875) [FAIL]
Assert.All() Failure: 253 out of 256 items in the collection did not pass.
[3]: Item: 4
Error: Assert.All() Failure: 9 out of 24 items in the collection did not pass.
[12]: Item: -1.40129846E-45
Error: Assert.Equal() Failure: Values differ
Expected: NaN
Actual: -∞
The actual test is comparing the result of the TensorPrimitives.ReciprocalSqrtEstimate function against the in-box float.ReciprocalSqrtEstimate API. On .NET 8, the float version will call into Sse.ReciprocalSqrtScalar which treats subnormal values as flush-to-zero and therefore does 1 / Sqrt(-0.0f) which returns -∞, however the expected value is NaN indicated that the float version isntead called into Avx512F.Reciprocal14SqrtScalar which handles all non-zero negatives as properly negative and thus is doing 1 / Sqrt(nonZeroNegative) which returns NaN. This highlights that it is actually running against .NET 9 and not testing what we think it should be testing.