hostagent/dns: fix panic in TestDNSRecords on Windows

[alexandear]

This PR fixes the panic in TestDNSRecords. This happened during the run:

=== RUN   TestDNSRecords
    dns_test.go:39: 
--- FAIL: TestDNSRecords (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x0 pc=0x62a833]

goroutine 6 [running]:
testing.tRunner.func1.2({0x67cee0, 0x913da0})
	C:/hostedtoolcache/windows/go/1.21.3/x64/src/testing/testing.go:1545 +0x238
testing.tRunner.func1()
	C:/hostedtoolcache/windows/go/1.21.3/x64/src/testing/testing.go:1548 +0x397
panic({0x67cee0?, 0x913da0?})
	C:/hostedtoolcache/windows/go/1.21.3/x64/src/runtime/panic.go:914 +0x21f
github.com/foxcpp/go-mockdns.(*Server).Close(0x0)
	C:/Users/runneradmin/go/pkg/mod/github.com/foxcpp/go-mockdns@v1.0.0/server.go:414 +0x13
runtime.Goexit()
	C:/hostedtoolcache/windows/go/1.21.3/x64/src/runtime/panic.go:523 +0x145
testing.(*common).SkipNow(0xc00005b520)
	C:/hostedtoolcache/windows/go/1.21.3/x64/src/testing/testing.go:1114 +0x45
testing.(*common).Skip(0xc00005b520, {0x0?, 0xc000034d50?, 0x0?})
	C:/hostedtoolcache/windows/go/1.21.3/x64/src/testing/testing.go:1090 +0x54
github.com/lima-vm/lima/pkg/hostagent/dns.TestDNSRecords(0xc00005b520)
	C:/a/lima/lima/pkg/hostagent/dns/dns_test.go:39 +0x391
testing.tRunner(0xc00005b520, 0x6ff4a0)
	C:/hostedtoolcache/windows/go/1.21.3/x64/src/testing/testing.go:1595 +0xff
created by testing.(*T).Run in goroutine 1
	C:/hostedtoolcache/windows/go/1.21.3/x64/src/testing/testing.go:1648 +0x3ad
FAIL	github.com/lima-vm/lima/pkg/hostagent/dns	0.045s

[AkihiroSuda]

Thanks