test: Drop stdout polling from run-tests subprocesses
test: Drop stdout polling from run-tests subprocesses

Replace it with writing into an anonymous temporary file, so that the
test can write directly into it without having to worry about blocking
or polling.

If we ever need it for debugging/inspecting running tests, we could even
use named temporary files.

Closes #13356
cockpit-project/cockpit,cockpituous/cockpit,deryni/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,deryni/cockpit,garrett/cockpit,garrett/cockpit,garrett/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,mvollmer/cockpit,deryni/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,deryni/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit,cockpituous/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpituous/cockpit,mvollmer/cockpit