Add tests for toBeSlug method

tests/Features/Expect/toBeSlug.php

@@ -0,0 +1,24 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('pass', function () {
+    expect('This is a Test String!')->toBeSlug()
+        ->and('Another Test String')->toBeSlug();
+});
+
+test('failures', function () {
+    expect('')->toBeSlug();
+})->throws(ExpectationFailedException::class);
+
+test('failures with custom message', function () {
+    expect('')->toBeSlug('oh no!');
+})->throws(ExpectationFailedException::class, 'oh no!');
+
+test('failures with default message', function () {
+    expect('')->toBeSlug();
+})->throws(ExpectationFailedException::class, 'Failed asserting that  can be converted to a slug.');
+
+test('not failures', function () {
+    expect('This is a Test String!')->not->toBeSlug();
+})->throws(ExpectationFailedException::class);