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 <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>