One of the things that ClangRunnerTest is doing is capturing
stderr/stdout because clang prints to it directly. This adds support for
that to FileTest.
I'm renaming the current `capture_output` field to `dump_output` because
the name is ambiguous after this change, and the flag is already named
`--dump_output`. It's still not great, but at least it's more distinct.
Note ClangRunner still doesn't use the vfs; that still needs work. I'm
just moving the NoArgs test over as a trivial test of the functionality.
