Apply new limit to new users

I refactored the SubscribersFeature class so that it's easily
unit tested. Computing the subscribers count in the constructor
is better then computing it on every call to check().

[MAILPOET-2394]

tests/unit/Util/License/Features/SubscribersTest.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace MailPoet\Test\Util\License\Features;
+
+use Codeception\Util\Stub;
+use MailPoet\Util\License\Features\Subscribers as SubscribersFeature;
+
+class SubscribersTest extends \MailPoetUnitTest {
+
+  function testCheckReturnsTrueIfOldUserReachedLimit() {
+    $subscribers_feature = Stub::make(SubscribersFeature::class, [
+      'license' => false,
+      'installation_time' => strtotime('2018-11-11'),
+      'subscribers_count' => 2500,
+    ]);
+    expect($subscribers_feature->check())->true();
+  }
+
+  function testCheckReturnsFalseIfOldUserDidntReachLimit() {
+    $subscribers_feature = Stub::make(SubscribersFeature::class, [
+      'license' => false,
+      'installation_time' => strtotime('2018-11-11'),
+      'subscribers_count' => 1500,
+    ]);
+    expect($subscribers_feature->check())->false();
+  }
+
+  function testCheckReturnsTrueIfNewUserReachedLimit() {
+    $subscribers_feature = Stub::make(SubscribersFeature::class, [
+      'license' => false,
+      'installation_time' => strtotime('2019-11-11'),
+      'subscribers_count' => 1500,
+    ]);
+    expect($subscribers_feature->check())->true();
+  }
+
+  function testCheckReturnsFalseIfNewUserDidntReachLimit() {
+    $subscribers_feature = Stub::make(SubscribersFeature::class, [
+      'license' => false,
+      'installation_time' => strtotime('2019-11-11'),
+      'subscribers_count' => 900,
+    ]);
+    expect($subscribers_feature->check())->false();
+  }
+
+  function testCheckReturnsFalseIfLicenseExists() {
+    $subscribers_feature = Stub::make(SubscribersFeature::class, [
+      'license' => true,
+      'installation_time' => strtotime('2019-11-11'),
+      'subscribers_count' => 1500,
+    ]);
+    expect($subscribers_feature->check())->false();
+  }
+}