make asset_test.dart tests not dependent on context (#141331)

Part of work on https://github.com/flutter/flutter/issues/141330, which is a part of work on https://github.com/flutter/flutter/issues/140092

This is a refactoring; there should be no behavioral changes in these tests.

packages/flutter_tools/test/data/asset_test/font/pubspec.yaml

-name: font
-description: A test project that contains a font.
-
-environment:
-  sdk: '>=3.2.0-0 <4.0.0'
-
-flutter:
-  uses-material-design: true
-  fonts:
-   - family: test_font
-     fonts:
-       - asset: test_font_file

packages/flutter_tools/test/data/asset_test/font/test_font_file

-This is a fake font.

packages/flutter_tools/test/data/asset_test/main/.gitignore

-.packages

packages/flutter_tools/test/data/asset_test/main/pubspec.yaml

-name: main
-description: A test project that has a package with a font as a dependency.
-
-environment:
-  sdk: '>=3.2.0-0 <4.0.0'
-
-dependencies:
-  font:
-    path: ../font
-
-# PUBSPEC CHECKSUM: 0000

packages/flutter_tools/test/data/fuchsia_test/main/.gitignore

-!.packages
\ No newline at end of file