t: prepare `test_when_finished ()`/`test_atexit()` for `set -e`

t: prepare `test_when_finished ()`/`test_atexit()` for `set -e`

Both `test_when_finished ()` and `test_atexit ()` build up a chain of
cleanup commands by prepending each new command to the existing cleanup
string. To preserve the exit code of the test body across cleanup
execution, we append the following logic:

    } && (exit "$eval_ret"); eval_ret=$?; ...

The intent of this is to run the cleanup block and then unconditionally
restore `eval_ret`. The original behaviour of this is is:

   +------------------+---------+------------------------------------+
   |test body         │ cleanup │ old behaviour                      │
   +------------------+---------+------------------------------------+
   │pass (eval_ret=0) | pass    │ && taken -> (exit 0) -> eval_ret=0 |
   +------------------+---------+------------------------------------+
   │pass (eval_ret=0) | fail    │ && not taken -> eval_ret=$?        |
   +------------------+---------+------------------------------------+
   │fail (eval_ret=1) | pass    │ && taken -> (exit 1) -> eval_ret=1 |
   +------------------+---------+------------------------------------+
   │fail (eval_ret=1) | fail    | && not taken -> eval_ret=$?        |
   +------------------+---------+------------------------------------+

This logic will start to fail once we enable `set -e`. When `$eval_ret`
is non-zero, the subshell we create will fail, and with `set -e` we'll
thus bail out without evaluating the logic after the semicolon.

Fix this issue by instead using `|| eval_ret=\$?; ...`. Besides being
a bit simpler, it also retains the original behaviour:

   +------------------+---------+------------------------------------+
   |test body         │ cleanup │ old behaviour                      │
   +------------------+---------+------------------------------------+
   │pass (eval_ret=0) | pass    │ || not taken -> eval_ret unchanged |
   +------------------+---------+------------------------------------+
   │pass (eval_ret=0) | fail    │ || taken -> eval_ret=$?            |
   +------------------+---------+------------------------------------+
   │fail (eval_ret=1) | pass    │ || not taken -> eval_ret unchanged |
   +------------------+---------+------------------------------------+
   │fail (eval_ret=1) | fail    | || taken -> eval_ret=$?            |
   +------------------+---------+------------------------------------+

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>