Make it easy to test Generators derived from package:source_gen by annotating test files.

@ShouldGenerate(
  r'''
const TestClass1NameLength = 10;

const TestClass1NameLowerCase = testclass1;
''',
  configurations: ['default', 'no-prefix-required'],
)
@ShouldThrow(
  'Uh...',
  configurations: ['vague'],
  element: false,
)
@TestAnnotation()
class TestClass1 {}

Other helpers are also provided.