Make a separate cmake option for caffe2 tests (#37721)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37721
Even though we disabled caffe2 test configs in Python, the BUILD_TEST
option was still building caffe2 test cpp binaries and various CI
configurations were running them (since they just run every binary in
`torch/test`).
This PR adds a caffe2-specific BUILD_TEST option (BUILD_CAFFE2_TEST),
which defaults to OFF, and gates the compilation of caffe2 test cpp
binaries under it.
Test Plan: Imported from OSS
Differential Revision: D21369541
Pulled By: suo
fbshipit-source-id: 669cff70c5b53f016e8e016bcb3a99bf3617e1f9
