t1020: avoid aborting entire test script when one test fails

t1020: avoid aborting entire test script when one test fails

Although `exit 1` is the proper way to signal a test failure from within
a subshell, its use outside any subshell should be avoided since it
aborts the entire script rather than aborting only the failed test.
Instead, a simple `return 1` is the proper idiom for signaling failure
outside a subshell since it aborts only the test in question, not the
entire script.

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