tests: add extra protection against unexpected exits

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.

tests/init.sh

@@ -93,6 +93,19 @@ skip_ () { warn_ "$ME_: skipped test: $@"; Exit 77; }
 fatal_ () { warn_ "$ME_: hard error: $@"; Exit 99; }
 framework_failure_ () { warn_ "$ME_: set-up failure: $@"; Exit 99; }
 
+# This is used to simplify checking of the return value
+# which is useful when ensuring a command fails as desired.
+# I.E. just doing `command ... &&fail=1` will not catch
+# a segfault in command for example.  With this helper you
+# instead check an explicit exit code like
+#   returns_ 1 command ... || fail
+returns_ () {
+  local exp_exit="$1"
+  shift
+  "$@"
+  test $? -eq $exp_exit
+}
+
 # Sanitize this shell to POSIX mode, if possible.
 DUALCASE=1; export DUALCASE
 if test -n "${ZSH_VERSION+set}" && (emulate sh) >/dev/null 2>&1; then