Strip CORS headers from applications (#8057)

The problem is that the headers get doubled up (not overwritten) and
browsers do not like multiple values for the allowed origin even though
it appears the spec allows for it.

We could prefer the application's headers instead of ours but since we
control OPTIONS I think preferring ours will by the more consistent
experience and also aligns with the original RFC.

coderd/workspaceapps/apptest/setup.go

@@ -52,6 +52,7 @@ type DeploymentOptions struct {
 	// The following fields are only used by setupProxyTestWithFactory.
 	noWorkspace bool
 	port        uint16
+	headers     http.Header
 }
 
 // Deployment is a license-agnostic deployment with all the fields that apps
@@ -184,7 +185,7 @@ func setupProxyTestWithFactory(t *testing.T, factory DeploymentFactory, opts *De
 	}
 
 	if opts.port == 0 {
-		opts.port = appServer(t)
+		opts.port = appServer(t, opts.headers)
 	}
 	workspace, agnt := createWorkspaceWithApps(t, deployment.SDKClient, deployment.FirstUser.OrganizationID, me, opts.port)
 
@@ -233,7 +234,7 @@ func setupProxyTestWithFactory(t *testing.T, factory DeploymentFactory, opts *De
 	return details
 }
 
-func appServer(t *testing.T) uint16 {
+func appServer(t *testing.T, headers http.Header) uint16 {
 	// Start a listener on a random port greater than the minimum app port.
 	var (
 		ln      net.Listener
@@ -261,6 +262,11 @@ func appServer(t *testing.T) uint16 {
 			_, err := r.Cookie(codersdk.SessionTokenCookie)
 			assert.ErrorIs(t, err, http.ErrNoCookie)
 			w.Header().Set("X-Forwarded-For", r.Header.Get("X-Forwarded-For"))
+			for name, values := range headers {
+				for _, value := range values {
+					w.Header().Add(name, value)
+				}
+			}
 			w.WriteHeader(http.StatusOK)
 			_, _ = w.Write([]byte(proxyTestAppBody))
 		}),