(#22649) fixture sources for example /proc/cpuinfo files should be consolidated

Merge the `/proc/cpuinfo` examples under `spec/fixtures/cpuinfo/` and
`spec/fixtures/unit/physialprocessorcount/` into
`spec/fixtures/cpuinfo/` and add a common set of access methods named
`#cpuinfo_fixture_read` and `#cpuinfo_fixture_readlines`