internal/plugintest: Switch from (os.File).Readdir() to os.ReadDir() (#1056)
internal/plugintest: Switch from (os.File).Readdir() to os.ReadDir() (#1056)

Reference: https://pkg.go.dev/os#File.Readdir
Reference: https://pkg.go.dev/os#ReadDir

While attempting to troubleshoot macOS kernel panic behaviors while running acceptance testing, I was able to capture a log whose last entry was `Symlinking source directories to work directory`. Since that operation tends to occur in temporary directory space, there is at least some potential there for strange macOS and Go behaviors.

The `(os.File).Readdir()` method does make this mention in particular:

> Most clients are better served by the more efficient ReadDir method.

Making this a branch so others potentially affected by macOS kernel panics can try this out. This may not be the root cause, as there is a lot more process-oriented logic that occurs to start and stop providers in the testing framework, but if nothing else it could help performance slightly.
hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk