Add more first-class Span tests

[jjonescz]

Test plan: #73445

[jjonescz]

@cston @333fred for test-only review, thanks

[cston]

public static implicit operator C(string s) => new C();

Does this overload have any effect? (Or should the argument be string[] instead?)

[333fred]

When we're doing a retargeting test, it's usually good practice to explicitly specify the underlying TFM for the second compilation, so we can be sure that it's a different TFM from the assembly that will be retargeted.