support accepting files in analysis_test.targets

One may want to specify:

```
analysis_test(
name = "my_test",
targets = ["foo.o"],
```

But currently `targets` does not support files.
At first glance, this would seem easy to implement, using `attr.label_list.allow_files`, however, this approach poses a different problem: one can then specify a file that does not actually exist, and the analysis_test thus passes.
Thus, the above example test might fail if the rule generating `foo.o` fails, but the following would pass:

```
analysis_test(
name = "my_test",
targets = ["foo.o_oopsfatfinger"],
```

We'd need some way to ensure an input file actually exists to implement this feature.

Start by locating the implementation of analysis_test and how its targets attribute is declared; the issue specifically mentions attr.label_list.allow_files. Investigate how the rule can distinguish an existing generated file from a misspelled target, and consider the examples in the issue as acceptance cases: valid files should be accepted while nonexistent files should fail.

Written by the indexing model from the issue text.