compiletest --nocapture still delays prints

[lolbinarycat]

add the following snippet to the top of any run-make test's main function:

   #![allow(unreachable_code)]
    println!("infinite loop");
    loop {}

then run COMPILETEST_FORCE_STAGE0=1 ./x test --stage 0 tests/run-make/TEST_NAME/ -- --nocapture

this should print "infinite loop" and then hang, but it does not print anything.

running the same piece of code on the playground works properly.

flushing stdout doesn't help either.