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/ln/misc.sh

@@ -52,20 +52,20 @@ rm -rf $d $f
 touch $f || framework_failure_
 rm -rf $d || framework_failure_
 mkdir $d $d/$f || framework_failure_
-ln $f $d/ 2> /dev/null && fail=1
-ln -s $f $d/ 2> /dev/null && fail=1
+returns_ 1 ln $f $d/ 2> /dev/null || fail=1
+returns_ 1 ln -s $f $d/ 2> /dev/null || fail=1
 rm -rf $d $f
 
 # Make sure we get a failure with existing dest without -f option
 touch $t || framework_failure_
 # FIXME: don't ignore the error message but rather test
 # it to make sure it's the right one.
-ln -s $t $t 2> /dev/null && fail=1
+returns_ 1 ln -s $t $t 2> /dev/null || fail=1
 rm $t
 
 # Make sure -sf fails when src and dest are the same
 touch $t || framework_failure_
-ln -sf $t $t 2> /dev/null && fail=1
+returns_ 1 ln -sf $t $t 2> /dev/null || fail=1
 rm $t
 
 # Create a symlink with source file and no explicit directory

tests/ln/slash-decorated-nonexistent-dest.sh

@@ -23,7 +23,7 @@ touch f || framework_failure_
 
 
 # Before coreutils-7.6, this would succeed on Solaris 10
-ln -T f no-such-file/ && fail=1
+returns_ 1 ln -T f no-such-file/ || fail=1
 test -e no-such-file && fail=1
 
 Exit $fail