Add test for the same error on every failed login

test/cypress/e2e/api/Tokens.cy.js

@@ -0,0 +1,61 @@
+/// <reference types="cypress" />
+
+describe('Tokens endpoints', () => {
+	const user = {
+		email:  'cypress@example.com',
+		secret: 'changeme',
+	};
+	const disabledUser = {
+		email:  'disabled@example.com',
+		secret: 'changeme',
+	};
+	const unknownEmail = 'nobody@example.com';
+
+	const login = (identity, secret) => {
+		return cy.request({
+			method:           'POST',
+			url:              '/api/tokens',
+			body:             {identity, secret},
+			failOnStatusCode: false,
+		});
+	};
+
+	before(() => {
+		cy.resetUsers();
+		cy.getToken().then((token) => {
+			cy.task('backendApiPost', {
+				token: token,
+				path:  '/api/users',
+				data:  {
+					name:        'Disabled User',
+					nickname:    'Disabled',
+					email:       disabledUser.email,
+					is_disabled: true,
+					auth:        {
+						type:   'password',
+						secret: disabledUser.secret,
+					},
+				},
+			});
+		});
+	});
+
+	it('Should give the same answer for an unknown email, a disabled account and a wrong password', () => {
+		// Only the error is compared: CI runs the backend in debug mode, which adds a stack trace that a real
+		// client never gets.
+		login(user.email, 'not-the-password').then((wrongPassword) => {
+			expect(wrongPassword.status).to.equal(400);
+			expect(wrongPassword.body.error.message_i18n).to.equal('error.invalid-auth');
+
+			login(unknownEmail, user.secret).then((unknown) => {
+				expect(unknown.status).to.equal(wrongPassword.status);
+				expect(unknown.body.error).to.deep.equal(wrongPassword.body.error);
+			});
+
+			login(disabledUser.email, disabledUser.secret).then((disabled) => {
+				expect(disabled.status).to.equal(wrongPassword.status);
+				expect(disabled.body.error).to.deep.equal(wrongPassword.body.error);
+			});
+		});
+	});
+});