Delete unused file closure/private/rule_test.bzl (#493)
Delete unused file closure/private/rule_test.bzl (#493)

I haven't found any use of the file neither in this repository, nor at
Google. The file uses `ctx.new_file` which is deprecated (calling it is
an error since Bazel 0.27). Instead of fixing it, I opted for deleting
the file.
bazelbuild/rules_closure,bazelbuild/rules_closure,gramic/rules_closure,gramic/rules_closure,bazelbuild/rules_closure,gramic/rules_closure,bazelbuild/rules_closure,gramic/rules_closure