feat: use default org for PostUser (#12143)

Instead of assuming only 1 org exists, this uses the
is_default org to place a user in if not specified.

coderd/users_test.go

@@ -493,21 +493,26 @@ func TestPostUsers(t *testing.T) {
 		t.Parallel()
 		auditor := audit.NewMock()
 		client := coderdtest.New(t, &coderdtest.Options{Auditor: auditor})
-		numLogs := len(auditor.AuditLogs())
-
 		firstUser := coderdtest.CreateFirstUser(t, client)
-		numLogs++ // add an audit log for user create
-		numLogs++ // add an audit log for login
 
 		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
 		defer cancel()
 
+		// Add an extra org to try and confuse user creation
+		_, err := client.CreateOrganization(ctx, codersdk.CreateOrganizationRequest{
+			Name: "foobar",
+		})
+		require.NoError(t, err)
+
+		numLogs := len(auditor.AuditLogs())
+
 		user, err := client.CreateUser(ctx, codersdk.CreateUserRequest{
 			Email:    "another@user.org",
 			Username: "someone-else",
 			Password: "SomeSecurePassword!",
 		})
 		require.NoError(t, err)
+		numLogs++ // add an audit log for user create
 
 		require.Len(t, auditor.AuditLogs(), numLogs)
 		require.Equal(t, database.AuditActionCreate, auditor.AuditLogs()[numLogs-1].Action)