test: fix non-constant Errorf format strings

[mikelolasagasti]

This change will solve future problems with newer Go releases:

go test ./...
# github.com/adnanh/webhook/internal/hook
# [github.com/adnanh/webhook/internal/hook]
internal/hook/hook_test.go:437:13: non-constant format string in call to (*testing.common).Errorf
internal/hook/hook_test.go:454:13: non-constant format string in call to (*testing.common).Errorf
ok  	github.com/adnanh/webhook	16.256s
FAIL	github.com/adnanh/webhook/internal/hook [build failed]
?   	github.com/adnanh/webhook/internal/middleware	[no test files]
ok  	github.com/adnanh/webhook/internal/pidfile	0.005s
?   	github.com/adnanh/webhook/test	[no test files]
FAIL