Stop using sed -i, as it doesn't seem portable.

On BSD systems like OS X, sed -i '' disables saving backup files, but
this doesn't work with GNU sed. GNU sed's way of disabling backup files
seems to be to just not provide an argument to -i, which doesn't work
with BSD sed. Instead, just refactor test code so that sed -i isn't
used.

test/test-extract

@@ -211,10 +211,9 @@ lha_check_output() {
 	local output="$wd/output.txt"
 
 	cd "$run_sandbox"
-	test_lha "$@" >"$output" 2>&1
-
+	# Invoke test command and save output.
 	# test outputs have their test root as '/tmp'; adjust accordingly
-	sed -i '' -e "s,$wd,/tmp,g" "$output"
+	test_lha "$@" 2>&1 | sed "s,$wd,/tmp,g" > "$output"
 
 	cd "$test_base"