Deprecated ctx.outputs.executable used in rules/test_rule

In ```line_length.bzl```:
```
ctx.actions.write(
output = ctx.outputs.executable,
content = script,
)
```
Notice that ```ctx.outputs.executable``` default field is used, and the [documentation](https://docs.bazel.build/versions/2.0.0/skylark/lib/ctx.html#outputs) states this is deprecated:

> (Deprecated) If the rule is marked executable or test, there is a field named "executable", which is the default executable. It is recommended that instead of using this, you pass another file (either predeclared or not) to the executable arg of DefaultInfo.