[5.8] Add assertSessionHasInput to TestResponse

[jasonmccreary]

This adds an assertSessionHasInput() helper method to the TestReponse class. Inline with the other session assertions, it accepts an array or a key value pair to verify exists in the flashed session array input.

Although you can test this with the available assertions, this new assertion avoids writing brittle tests which rely on internal framework values like the _old_input key.

$response->assertSessionHas('_old_input', [
    'connection_id' => $connection->id,
    'repository' => $repository,
    'source_branch' => $branch,
]);

Instead, you can now write:

$response->assertSessionHasInput([
    'connection_id' => $connection->id,
    'repository' => $repository,
    'source_branch' => $branch,
]);

[jasonmccreary]

If merged, I will submit a PR to update the docs as well by adding this to the list of Available Assertions.

[SjorsO]

I think it would be useful if the assertions prints the actual old input when the assertion fails, similar to how it happens in this pr

[jasonmccreary]

@SjorsO, I'm not sure the difference in the output this would have. Maybe you could share the suggested code or could PR it if this were merged.