t: fix &&-chaining issues around setup which might fail

Many tests have an initial setup step that might fail based
on whether earlier tests in the script have succeeded or
not. Using a trick like "|| true" breaks the &&-chain,
missing earlier failures (and fooling --chain-lint).

We can use test_might_fail in some cases, which is correct
and makes the intent more obvious. We can also use
test_unconfig for unsetting config (and which is more
robust, as well).

The case in t9500 is an oddball. It wants to run cmd1 _or_
cmd2, and does it like:

  cmd1 || cmd2 &&
  other_stuff

It's not wrong in this case, but it's a bad habit to get
into, because it breaks the &&-chain if used anywhere except
at the beginning of the test (and we use the correct
solution here, putting it inside a block for precedence).

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

t/t7201-co.sh

@@ -591,7 +591,7 @@ test_expect_success 'checkout --conflict=merge, overriding config' '
 '
 
 test_expect_success 'checkout --conflict=diff3' '
-	git config --unset merge.conflictstyle
+	test_unconfig merge.conflictstyle &&
 	setup_conflicting_index &&
 	echo "none of the above" >sample &&
 	echo ourside >expect &&