Update Composer config in PHP module (#3829)

* Prevents installation of `phpunit/phpunit:^6` because the test cases in this project require `\PHPUnit_Framework_TestCase` which was removed in v6. Moves generated PHP classes for tests into autoload-dev so that they are not referenced in the dist autoload. Adds Composer scripts to run protoc with and without grpc.

* update composer test target to use the same commands as generate_php_test_proto in tests.sh

php/composer.json

@@ -9,13 +9,20 @@
     "php": ">=5.5.0"
   },
   "require-dev": {
-    "phpunit/phpunit": ">=4.8.0"
+    "phpunit/phpunit": "^5|^4.8.0"
   },
   "autoload": {
     "psr-4": {
       "Google\\Protobuf\\": "src/Google/Protobuf",
-      "GPBMetadata\\Google\\Protobuf\\": "src/GPBMetadata/Google/Protobuf",
+      "GPBMetadata\\Google\\Protobuf\\": "src/GPBMetadata/Google/Protobuf"
+    }
+  },
+  "autoload-dev": {
+    "psr-4": {
       "": "tests/generated"
     }
+  },
+  "scripts": {
+    "test": "(cd tests && rm -rf generated && mkdir -p generated && ../../src/protoc --php_out=generated proto/empty/echo.proto proto/test.proto proto/test_include.proto proto/test_no_namespace.proto proto/test_prefix.proto proto/test_php_namespace.proto proto/test_empty_php_namespace.proto proto/test_reserved_enum_lower.proto proto/test_reserved_enum_upper.proto proto/test_reserved_enum_value_lower.proto proto/test_reserved_enum_value_upper.proto proto/test_reserved_message_lower.proto proto/test_reserved_message_upper.proto proto/test_service.proto proto/test_service_namespace.proto proto/test_descriptors.proto) && (cd ../src && ./protoc --php_out=../php/tests/generated -I../php/tests -I. ../php/tests/proto/test_import_descriptor_proto.proto) && vendor/bin/phpunit"
   }
 }