PR: Fix example plugin test build failure

[leiless]

Detailed test failure:

$ go test -v                                  
# _/tmp/example [_/tmp/example.test]
./example_test.go:33:44: undefined: example
FAIL	_/tmp/example [build failed]

After fix:

$ go test -v
=== RUN   TestExample
--- PASS: TestExample (0.00s)
=== RUN   TestSetup
--- PASS: TestSetup (0.00s)
PASS
ok  	_/home/lynnl/coredns-dev/example	0.011s