`test-bot` places log files and build outputs in the current working
directory. When testing a tap, this happened to be a tap directory that
lives in `$(brew --repo)/Library/Taps` and caused occasional sandbox
violations (e.g. in `post_install`). Avoid this by always running
`test-bot` from the main Homebrew repository root (like Jenkins).
Without the trailing `/`, rsync will create an unnecessary folder over
the tap path, which leads the filesystem to something like:
Library/Taps/homebrew/homebrew-core/homebrew-core/...
This is a partial fix for https://github.com/Homebrew/brew/issues/100
