Revert "tests: Fix GOptionContext leak in GSubprocess tests"

This reverts commit 52bab025.

It silently conflicted with another commit, 90ca3b4d, which was merged later than it. I’ve kept commit 90ca3b because it also frees the GError; 52bab doesn’t.

This is my failure to rebase and test old branches before merging them, instead of assuming that the lack of automatically detected merge conflicts actually means there are no merge conflicts.