More consistent temp directory naming (#42063)

Applying our style guide:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#have-good-hygiene-when-using-temporary-directories
diff --git a/packages/flutter_tools/test/general.shard/plugins_test.dart b/packages/flutter_tools/test/general.shard/plugins_test.dart
index 1528828..073fee4 100644
--- a/packages/flutter_tools/test/general.shard/plugins_test.dart
+++ b/packages/flutter_tools/test/general.shard/plugins_test.dart
@@ -197,7 +197,7 @@
       when(androidProject.appManifestFile).thenReturn(androidManifest);
 
       final Directory pluginUsingJavaAndNewEmbeddingDir =
-        fs.systemTempDirectory.createTempSync('pluginUsingJavaAndNewEmbeddingDir.');
+        fs.systemTempDirectory.createTempSync('flutter_plugin_using_java_and_new_embedding_dir.');
       pluginUsingJavaAndNewEmbeddingDir
         .childFile('pubspec.yaml')
         .writeAsStringSync('''
@@ -217,7 +217,7 @@
         ..writeAsStringSync('import io.flutter.embedding.engine.plugins.FlutterPlugin;');
 
       final Directory pluginUsingKotlinAndNewEmbeddingDir =
-        fs.systemTempDirectory.createTempSync('pluginUsingKotlinAndNewEmbeddingDir.');
+        fs.systemTempDirectory.createTempSync('flutter_plugin_using_kotlin_and_new_embedding_dir.');
       pluginUsingKotlinAndNewEmbeddingDir
         .childFile('pubspec.yaml')
         .writeAsStringSync('''
@@ -237,7 +237,7 @@
         ..writeAsStringSync('import io.flutter.embedding.engine.plugins.FlutterPlugin');
 
       final Directory pluginUsingOldEmbeddingDir =
-        fs.systemTempDirectory.createTempSync('pluginUsingOldEmbeddingDir.');
+        fs.systemTempDirectory.createTempSync('flutter_plugin_using_old_embedding_dir.');
       pluginUsingOldEmbeddingDir
         .childFile('pubspec.yaml')
         .writeAsStringSync('''