* create `repotest.WritePluginCallback` function
* add test for `list-bin-paths` callback that returns non-existent path
* if directory name returned by `list-bin-paths` doesn't exist skip it
Some of the Go tests actually cloned remote repos, which meant slow tests, and tests failures if I was working offline. These changes allow all Go tests to run offline. The test code now clones local Git repos instead of remotes ones.
* Add otiai10/copy as a dependency
* Create repotest package for test code that needs to work with Git repos
* Add back missing test script for execute package
* Update git, pluginindex, and plugins package tests so they work offline