Proposal for golden output testing rule

In #86 it appears the first rule is being added to skylib. Would there be any interest in adding a new rule that does golden comparison? To pick a name let's call it `golden_test`. I propose it is usable in 3 ways:

Literal:

```
golden_test(
name = "content",
src = ":myfile.txt", # Note that this could be the output of a genrule or other target
content = "demonstration content for a test",
)
```

Regexp

```
golden_test(
name = "content",
src = ":myfile.txt", # Note that this could be the output of a genrule or other target
regexp = "demonstration content for a .*",
)
```

Golden file

```
golden_test(
name = "content",
src = ":myfile.txt", # Note that this could be the output of a genrule or other target
golden_file = ":myfile.golden.txt",
)
```

Would there be any interest in adding a rule like this? I believe that almost every rules_x has implemented something like this and consolidating on a single implementation seems like a not terrible idea.

CC: @c-parsons