Increase coverage

[pflorek]

Have for each construct at least a very simple test

For example matching a snapshot:

https://github.com/pepperize/cdk-github/blob/main/test/github-custom-resource.test.ts

describe("GithubCustomResource", () => {
  it("Should match snapshot", () => {
    // Given
    const stack = new Stack();
    new GithubCustomResource(stack, "CR", {
      // ...
    });
    
    // When
    const template = Template.fromStack(stack);
    
    // Then
    expect(template).toMatchSnapshot();
  });
});