vet fix

Fixes the following vet reports:

```
memberlist_test.go:342: possible formatting directive in Fatal call
memberlist_test.go:348: possible formatting directive in Fatal call
memberlist_test.go:351: possible formatting directive in Fatal call
memberlist_test.go:413: possible formatting directive in Fatal call
memberlist_test.go:419: possible formatting directive in Fatal call
memberlist_test.go:422: possible formatting directive in Fatal call
memberlist_test.go:465: possible formatting directive in Fatal call
memberlist_test.go:471: possible formatting directive in Fatal call
memberlist_test.go:474: possible formatting directive in Fatal call
memberlist_test.go:661: possible formatting directive in Fatal call
memberlist_test.go:665: possible formatting directive in Fatal call
memberlist_test.go:668: possible formatting directive in Fatal call
memberlist_test.go:721: possible formatting directive in Fatal call
memberlist_test.go:727: possible formatting directive in Fatal call
memberlist_test.go:730: possible formatting directive in Fatal call
memberlist_test.go:810: possible formatting directive in Fatal call
memberlist_test.go:837: possible formatting directive in Fatal call
memberlist_test.go:843: possible formatting directive in Fatal call
memberlist_test.go:846: possible formatting directive in Fatal call
memberlist_test.go:874: possible formatting directive in Fatal call
memberlist_test.go:891: possible formatting directive in Fatal call
memberlist_test.go:897: possible formatting directive in Fatal call
memberlist_test.go:900: possible formatting directive in Fatal call
```