various: add golangci-lint, fix issues (#7905)

This adds an initial and intentionally minimal configuration for
golang-ci, fixes the issues reported, and adds a GitHub Action to check
new pull requests against this linter configuration.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I8f38fbc315836a19a094d0d3e986758b9313f163

net/dns/manager_tcp_test.go

@@ -108,7 +108,7 @@ func TestDNSOverTCP(t *testing.T) {
 		"bradfitz.ts.com.": "2.3.4.5",
 	}
 
-	for domain, _ := range wantResults {
+	for domain := range wantResults {
 		b := mkDNSRequest(domain, dns.TypeA, addEDNS)
 		binary.Write(c, binary.BigEndian, uint16(len(b)))
 		c.Write(b)

net/dns/manager_windows_test.go

@@ -502,7 +502,7 @@ func genRandomSubdomains(t *testing.T, n int) []dnsname.FQDN {
 	for len(domains) < cap(domains) {
 		l := r.Intn(19) + 1
 		b := make([]byte, l)
-		for i, _ := range b {
+		for i := range b {
 			b[i] = charset[r.Intn(len(charset))]
 		}
 		d := string(b) + ".example.com"