tests: apply modern idiom for exiting loop upon failure

tests: apply modern idiom for exiting loop upon failure

Rather than maintaining a flag indicating a failure within a loop and
aborting the test when the loop ends if the flag is set, modern practice
is to signal the failure immediately by exiting the loop early via
`return 1` (or `exit 1` if inside a subshell). Simplify these loops by
following the modern idiom.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>