net/dns: add test for DoH upgrade of system DNS

Someone asked me if we use DNS-over-HTTPS if the system's resolver is an
IP address that supports DoH and there's no global nameserver set (i.e.
no "Override DNS servers" set). I didn't know the answer offhand, and it
took a while for me to figure it out. The answer is yes, in cases where
we take over the system's DNS configuration and read the base config, we
do upgrade any DoH-capable resolver to use DoH. Here's a test that
verifies this behaviour (and hopefully helps as documentation the next
time someone has this question).

Updates #cleanup

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

net/dns/publicdns/publicdns.go

@@ -13,12 +13,14 @@ import (
 	"log"
 	"math/big"
 	"net/netip"
+	"slices"
 	"sort"
 	"strconv"
 	"strings"
 	"sync"
 
 	"tailscale.com/feature/buildfeatures"
+	"tailscale.com/util/testenv"
 )
 
 // dohOfIP maps from public DNS IPs to their DoH base URL.
@@ -367,3 +369,39 @@ func IPIsDoHOnlyServer(ip netip.Addr) bool {
 		controlDv6RangeA.Contains(ip) || controlDv6RangeB.Contains(ip) ||
 		ip == controlDv4One || ip == controlDv4Two
 }
+
+var testMu sync.Mutex
+
+// RegisterTestDoHEndpoint registers a test DoH endpoint mapping for use in tests.
+// It maps the given IP to the DoH base URL, and the URL back to the IP.
+//
+// This function panics if called outside of tests, and cannot be called
+// concurrently with any usage of this package (i.e. before any DNS forwarders
+// are created). It is safe to call concurrently with itself.
+//
+// It returns a cleanup function that removes the registration.
+func RegisterTestDoHEndpoint(ip netip.Addr, dohBase string) func() {
+	if !testenv.InTest() {
+		panic("RegisterTestDoHEndpoint called outside of tests")
+	}
+	populateOnce.Do(populate)
+
+	testMu.Lock()
+	defer testMu.Unlock()
+
+	dohOfIP[ip] = dohBase
+	dohIPsOfBase[dohBase] = append(dohIPsOfBase[dohBase], ip)
+
+	return func() {
+		testMu.Lock()
+		defer testMu.Unlock()
+
+		delete(dohOfIP, ip)
+		dohIPsOfBase[dohBase] = slices.DeleteFunc(dohIPsOfBase[dohBase], func(addr netip.Addr) bool {
+			return addr == ip
+		})
+		if len(dohIPsOfBase[dohBase]) == 0 {
+			delete(dohIPsOfBase, dohBase)
+		}
+	}
+}