Use fake getent to avoid existing system users

The default user may or may not exist on the system running the tests.
Create a fake getent to simulate non-existing user in a consistent way.

fixes https://gitlab.alpinelinux.org/alpine/cloud/tiny-cloud/-/issues/42

tests/init-boot.test

@@ -114,6 +114,10 @@ enable_sshd_body() {
 }
 
 create_default_user_body() {
+	fake_bin getent <<-EOF
+		#!/bin/sh
+		exit 2
+	EOF
 	atf_check \
 		-o match:"addgroup alpine" \
 		-o match:"adduser.*alpine" \