Add support for installing Pest into a Lumen application

This entails creating Laravel and Lumen-specific stubs, and ensuring
that the appropriate stubs are copied as part of the pest install.

stubs/Lumen/Helpers.php

@@ -0,0 +1,11 @@
+<?php
+
+use Illuminate\Contracts\Auth\Authenticatable;
+
+/**
+ * Set the currently logged in user for the application.
+ */
+function actingAs(Authenticatable $user, string $driver = null): TestCase
+{
+    return test()->actingAs($user, $driver);
+}

stubs/Lumen/Pest.php

@@ -0,0 +1,3 @@
+<?php
+
+uses(TestCase::class);

stubs/Lumen/phpunit.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="./vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+>
+    <testsuites>
+        <testsuite name="Application Test Suite">
+            <directory suffix="Test.php">./tests</directory>
+        </testsuite>
+    </testsuites>
+    <filter>
+        <whitelist processUncoveredFilesFromWhitelist="true">
+            <directory suffix=".php">./app</directory>
+        </whitelist>
+    </filter>
+</phpunit>