RFC: Do not package tests by default

I was unaware until recently that conda-build will also package whatever is being used in (& copied into) the test section as part of the final binary.

In general, I consider it best-practice to run the full upstream test suite against the package builds (and I've been doing this as much as possible on the feedstocks I'm involved with), but it's really unfortunate that an otherwise really good practice will then lead to extra package bloat (e.g. https://github.com/conda-forge/pillow-feedstock/issues/117, https://github.com/conda-forge/scipy-feedstock/issues/160), as well as other potential issues (e.g. https://github.com/conda-forge/pillow-feedstock/issues/113)

I know that one of the features of conda is that one can invoke the tests of a given package from the CLI, but is that really a common usecase for packages by conda-forge? Why not use the conda-build feature `--no-copy-test-source-files` by default in our builds? Or perhaps coupled with a flag to switch it back on for those few cases that really need it.

Thoughts?