Add do-block support for redirect_std[out,err,in]

[timholy]

In particular, these simplify the process of safely testing for warnings, as exhibited in the tests:

open(filename, "w") do f
    redirect_stderr(f) do
        warn("hello")
    end
end
@test contains(readstring(filename), "WARNING: hello")

I assume this counts as a new feature and can't be backported? If so, perhaps I will add this to Compat. (If not, please add the "backport-pending" label.)

[tkelman]

better in Compat, since it should work for 0.4 if done in Compat, but it won't be backported to 0.4 at this point

[timholy]

I'm not personally worried about the lack of support for it in 0.4. My question really was about 0.5.

[vtjnash]

Fixes #7022?

[timholy]

I guess for that to happen I should add stdin to the list.

[stevengj]

Would be nice to have capture_stdout(f::Function) and capture_stderr, to automate the process of creating a temporary file, opening it, executing code, and reading the file, then deleting it, since we have to write this code anyway in the tests.