ipv4: add 'ipv4_spurious_leftover_route' test

Tests: NetworkManager!935 (closed) a fix for: https://bugzilla.redhat.com/show_bug.cgi?id=1979192

Dnsmasq is started multiple times so that:

• different address range is used each time
• any leftover IPs in routing table from previous times can be thus detected easily
• we don't have to wait for the first lease to expire