macros: fix #[tokio::main] without rt-core (#2139)

The Tokio runtime provides a "shell" runtime when `rt-core` is not
available. This shell runtime is enough to support `#[tokio::main`] and
`#[tokio::test].

A previous change disabled these two attr macros when `rt-core` was not
selected. This patch fixes this by re-enabling the `main` and `test`
attr macros without `rt-core` and adds some integration tests to prevent
future regressions.

tests-integration/tests/process_stdio.rs

@@ -1,4 +1,5 @@
 #![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
 
 use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
 use tokio::process::{Child, Command};