Add regression test to use RUSTFLAGS value in doctests

Author: GuillaumeGomez

tests/rustdoc-ui/doctest/rustflags.rs

@@ -0,0 +1,11 @@
+//@ check-pass
+//@ compile-flags: --test
+//@ rustc-env:RUSTFLAGS=--cap-lints=warn
+
+/// ```
+/// #![deny(warnings)]
+/// #![feature(async_await)]
+///
+/// let x = 12;
+/// ```
+pub struct Bar;

tests/rustdoc-ui/doctest/rustflags.stdout

@@ -0,0 +1,6 @@
+
+running 1 test
+test tests/rustdoc-ui/doctest/rustflags.rs - Bar (line 5) ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.06s
+