[PATCH] tests: use "returns_" rather than explicit comparison with "$?"

[Jim Meyering]

I noticed many tests that compare directly with "$?". However, we now
have the "returns_" function (from init.sh) that can be used to make
the resulting code a little less error-prone: all on one line/stmt, it
is harder to accidentally insert code that accidentally clobbers "$?".

Here's an example of what most of these changes look like:

  -ls -l --time-style=XX > out 2> err
  -test $? = 2 || fail=1
  +returns_ 2 ls -l --time-style=XX > out 2> err || fail=1

cu-use-returns_.diff
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