ipam: test ECMP support

[yushoyamaguchi]

When multiple routes entries share the same dst but list different gw values, they collectively enable ECMP (e.g. two default routes with distinct gateways).

      "routes": [
        { "dst": "0.0.0.0/0", "gw": "10.0.0.1" },
        { "dst": "0.0.0.0/0", "gw": "10.0.0.2" }
      ]

The implementation already handles this path—ConfigureIface programs both routes, and static IPAM returns them—so real traffic gets load-balanced across gateways today.
https://github.com/containernetworking/plugins/blob/v1.9.1/pkg/ipam/ipam_linux.go#L148
This commit adds explicit tests for this ECMP behavior.

The concrete contents of this commit is below.

• add static IPAM spec proving multiple routes with the same dst keep distinct gateways
• add ConfigureIface ECMP test that inspects netlink routes for both gateways

[yushoyamaguchi]

After this PR is merged, I want to add ECMP document to https://github.com/containernetworking/cni.dev

[yushoyamaguchi]

The result of test

yamaguchi@yamaguchi-cilium-dev:~/mship-oss/cni-plugins$ sudo env "PATH=$(go env GOPATH)/bin:/usr/local/go/bin:$PATH"     sh -c 'umask 0; PATH=$(pwd)/bin:$PATH go test -race ./pkg/ipam'
ok      github.com/containernetworking/plugins/pkg/ipam 1.130s
yamaguchi@yamaguchi-cilium-dev:~/mship-oss/cni-plugins$ go test ./plugins/ipam/static
ok      github.com/containernetworking/plugins/plugins/ipam/static      0.036s

[yushoyamaguchi]

FYI:
Old and new ECMP style:
https://github.com/torvalds/linux/blob/v6.19/Documentation/networking/ip-sysctl.rst?plain=1#L2518-L2534

[squeed]

Very nice, thanks!