Don't autoload test cases using composer.

Introduces a bootstrap file that ensures that the base test case classes
are loaded and available instead.

This reduces the number of warnings when running composer install.

composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "a3d39881eecda53834cbcb595100e00e",
+    "content-hash": "e15f86e41cc765cbdd319fc2cc5311fb",
     "packages": [
         {
             "name": "blueimp/jquery-file-upload",
@@ -4534,12 +4534,12 @@
             "version": "3.6.2",
             "source": {
                 "type": "git",
-                "url": "https://github.com/squizlabs/PHP_CodeSniffer.git",
+                "url": "https://github.com/PHPCSStandards/PHP_CodeSniffer.git",
                 "reference": "5e4e71592f69da17871dba6e80dd51bce74a351a"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/squizlabs/PHP_CodeSniffer/zipball/5e4e71592f69da17871dba6e80dd51bce74a351a",
+                "url": "https://api.github.com/repos/PHPCSStandards/PHP_CodeSniffer/zipball/5e4e71592f69da17871dba6e80dd51bce74a351a",
                 "reference": "5e4e71592f69da17871dba6e80dd51bce74a351a",
                 "shasum": ""
             },
@@ -5280,6 +5280,8 @@
         "ext-sodium": "*",
         "ext-bcmath": "*"
     },
-    "platform-dev": [],
+    "platform-dev": {
+        "ext-yaml": "*"
+    },
     "plugin-api-version": "2.6.0"
 }