using readFile instead of fileContents (or using indented strings) can
leave a trailing newline that causes build errors in systemd units and
has previously caused runtime errors in wireguard scripts. use
singleLineStr to strip a trailing newline if it exists, and to fail if
more than one is present.
After working on the last wireguard bump (#57534), we figured that it's
probably a good idea to have a basic test which confirms that a simple
VPN with wireguard still works.
This test starts two peers with a `wg0` network interface and adds a v4
and a v6 route that goes through `wg0`.