run-command tests: use "return", not "exit"

run-command tests: use "return", not "exit"

Change the "run-command" test helper to "return" instead of calling
"exit", see 338abb0f045 (builtins + test helpers: use return instead
of exit() in cmd_*, 2021-06-08)

Because we'd previously gotten past the SANITIZE=leak check by using
exit() here we need to move to "goto cleanup" pattern.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>