Add tests for Instant::now() and wasm32-unknown-unknown

[Darksonn]

I would like us to add tests to avoid cases such as #7319 in the future. We should have tests for cases such as:

• Create a runtime without feature="time" and run code should not panic.
• Create a runtime with feature="time" and run code should not panic.
• Create a runtime with feature="time" and call tokio::time::{sleep,timeout} should panic.

Probably also some tests for what happens when you access time-based metrics. Should that panic? It doesn't right now - it just returns 0, but it probably should.