Explicitly specify protoc plugin in Plugin test target

This change explicitly specifies the swift grpc ptotoc plugin in the
`Plugin` directory, so users without `.` in the path can run the test
target.

Also add some build artifacts from the Plugin directory to .gitignore.

.gitignore

@@ -3,3 +3,5 @@ project.xcworkspace
 xcuserdata
 .build
 third_party/**
+Plugin/Packages/**
+Plugin/protoc-*