Add more cloexec tests

[zebreus]

This PR adds wasix tests for CLOEXEC on pipes. It also adds tests around popen, because that was where I was encountering these issues.

The tests will pass after wasix-org/wasix-libc#99 is merged

[Copilot]

Pull request overview

Adds new WASIX regression tests around FD_CLOEXEC behavior on pipes (notably pipe2(O_CLOEXEC)) and adds a popen-focused reproduction to catch stdin/EOF hangs caused by leaked pipe FDs across exec/spawn.

Changes:

• Add a new tests/wasix/popen test case that exercises vfork vs popen behavior when writing to a child’s stdin.
• Extend the existing tests/wasix/cloexec test to include a pipe2(O_CLOEXEC) flag verification.
• Update cloexec/run.sh to add timeouts and run the new test entrypoint.

Reviewed changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/wasix/popen/run.sh	Builds and runs the new popen/vfork regression scenarios under timeouts.
tests/wasix/popen/main.c	Implements combined echo/shell/vfork/popen test binary, including a popen-like implementation.
tests/wasix/popen/.no-build	Switches popen test directory to self-build via run.sh.
tests/wasix/cloexec/run.sh	Adds timeouts and runs the new pipe2_cloexec_test entrypoint (also recompiles).
tests/wasix/cloexec/main.c	Adds a new pipe2_cloexec_test subcommand to assert FD_CLOEXEC is set.

[zebreus]

Marked as draft until wasix-org/wasix-libc#99 is merged

[marxin]

Marked as draft until wasix-org/wasix-libc#99 is merged

The PR got merged, can we rebase the tests now?