Run Matmul 8 bit tests on CPU builds (#24946)
### Description
It seems like https://github.com/microsoft/onnxruntime/pull/24509 added
a guard for the 8 bit Matmul tests that depends on an MLAS macro being
set to compile and run on CPUs but that guard itself was preventing the
inclusion of the MLAS header where the macro would have been set and so
Matmul 8 bit tests were not being compiled and run on CPU builds.
### Motivation and Context
Improve test coverage for CPU builds
