Remove duplicate testOutputsDirectory definition from integration_test package (#150224)

I noticed a getter by this name with the same implementation already exists in flutter_driver, with the same implementation as the initializer for this variable. This deletes the variable and instead exports the getter.

Note that technically this could be breaking, because this version is a mutable variable instead of just a getter. But, I think this was just a mistake.

Author: jakemac53

packages/integration_test/lib/integration_test_driver.dart

@@ -14,13 +14,7 @@ import 'package:path/path.dart' as path;
 
 import 'common.dart';
 
-/// Flutter Driver test output directory.
-///
-/// Tests should write any output files to this directory. Defaults to the path
-/// set in the FLUTTER_TEST_OUTPUTS_DIR environment variable, or `build` if
-/// unset.
-String testOutputsDirectory =
-    Platform.environment['FLUTTER_TEST_OUTPUTS_DIR'] ?? 'build';
+export 'package:flutter_driver/flutter_driver.dart' show testOutputsDirectory;
 
 /// The callback type to handle [Response.data] after the test
 /// succeeds.

packages/integration_test/lib/integration_test_driver_extended.dart

@@ -14,13 +14,7 @@ import 'package:path/path.dart' as path;
 
 import 'common.dart';
 
-/// Flutter Driver test output directory.
-///
-/// Tests should write any output files to this directory. Defaults to the path
-/// set in the FLUTTER_TEST_OUTPUTS_DIR environment variable, or `build` if
-/// unset.
-String testOutputsDirectory =
-    Platform.environment['FLUTTER_TEST_OUTPUTS_DIR'] ?? 'build';
+export 'package:flutter_driver/flutter_driver.dart' show testOutputsDirectory;
 
 /// The callback type to handle [Response.data] after the test
 /// succeeds.