derp/derphttp: don't link websockets other than on GOOS=js

Or unless the new "ts_debug_websockets" build tag is set.

Updates #1278

Change-Id: Ic4c4f81c1924250efd025b055585faec37a5491d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/deptest/deptest.go

@@ -13,15 +13,19 @@ import (
 	"path/filepath"
 	"regexp"
 	"runtime"
+	"slices"
 	"strings"
 	"testing"
+
+	"tailscale.com/util/set"
 )
 
 type DepChecker struct {
-	GOOS    string            // optional
-	GOARCH  string            // optional
-	BadDeps map[string]string // package => why
-	Tags    string            // comma-separated
+	GOOS     string            // optional
+	GOARCH   string            // optional
+	BadDeps  map[string]string // package => why
+	WantDeps set.Set[string]   // packages expected
+	Tags     string            // comma-separated
 }
 
 func (c DepChecker) Check(t *testing.T) {
@@ -55,6 +59,11 @@ func (c DepChecker) Check(t *testing.T) {
 			t.Errorf("package %q is not allowed as a dependency (env: %q); reason: %s", dep, extraEnv, why)
 		}
 	}
+	for dep := range c.WantDeps {
+		if !slices.Contains(res.Deps, dep) {
+			t.Errorf("expected package %q to be a dependency (env: %q)", dep, extraEnv)
+		}
+	}
 	t.Logf("got %d dependencies", len(res.Deps))
 }