Many tests use `program ... && fail=1` to ensure expected
error situations are indicated. However that would mask
an unexpected exit (like a crash). Therefore explicitly
check the expected exit code.
Note where error messages are also verified, the extra
protection is not added.
* tests/init.sh (returns_): A new helper function to
check the return code of a command, and used
throughout the tests.
* cfg.mk (sc_prohibit_and_fail_1): Add a syntax check
to avoid new instances of this issue.
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