tests: replace chainlint subshell with a function

To test that we don't break the &&-chain, test-lib.sh does something
like:

   (exit 117) && $test_commands

and checks that the result is exit code 117. We don't care what that
initial command is, as long as it exits with a unique code. Using "exit"
works and is simple, but is a bit expensive since it requires a subshell
(to avoid exiting the whole script!). This isn't usually very
noticeable, but it can add up for scripts which have a large number of
tests.

Using "return" naively won't work here, because we'd return from the
function eval-ing the snippet (and it wouldn't find &&-chain breakages).
But if we further push that into its own function, it does exactly what
we want, without extra subshell overhead.

According to hyperfine, this produces a measurable improvement when
running t3070 (which has 1800 tests, all of them quite short):

  'HEAD' ran
    1.09 ± 0.01 times faster than 'HEAD~1'

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

t/test-lib.sh

@@ -1086,6 +1086,10 @@ test_eval_ () {
 	return $test_eval_ret_
 }
 
+fail_117 () {
+	return 117
+}
+
 test_run_ () {
 	test_cleanup=:
 	expecting_failure=$2
@@ -1097,7 +1101,7 @@ test_run_ () {
 		trace=
 		# 117 is magic because it is unlikely to match the exit
 		# code of other programs
-		if test "OK-117" != "$(test_eval_ "(exit 117) && $1${LF}${LF}echo OK-\$?" 3>&1)"
+		if test "OK-117" != "$(test_eval_ "fail_117 && $1${LF}${LF}echo OK-\$?" 3>&1)"
 		then
 			BUG "broken &&-chain or run-away HERE-DOC: $1"
 		fi