tests: Add virnumamock

tests: Add virnumamock

All mocked functions are related to numactl/virNuma and rely only on
virsysfs, so the paths they touch can be nicely controlled.  And
because it is so nicely self-contained NUMA mock, it is named
numamock (instead of naming it after the test that will use it first).

We need top level API mock because some APIs might call libnuma
directly, e.g. virNumaIsAvailable(), virNumaGetMaxNode().

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>