t6120-describe: modernize the 'check_describe' helper

The 'check_describe' helper function runs 'git describe' outside of
'test_expect_success' blocks, with extra hand-rolled code to record
and examine its exit code.

Update this helper and move the 'git describe' invocation inside the
'test_expect_success' block.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6120-describe.sh

@@ -16,14 +16,12 @@ test_description='test describe'
 check_describe () {
 	expect="$1"
 	shift
-	R=$(git describe "$@" 2>err.actual)
-	S=$?
-	cat err.actual >&3
-	test_expect_success "describe $*" '
-	test $S = 0 &&
+	describe_opts="$@"
+	test_expect_success "describe $describe_opts" '
+	R=$(git describe $describe_opts 2>err.actual) &&
 	case "$R" in
 	$expect)	echo happy ;;
-	*)	echo "Oops - $R is not $expect";
+	*)	echo "Oops - $R is not $expect" &&
 		false ;;
 	esac
 	'