Add regression test to use RUSTFLAGS value in doctests

Author: GuillaumeGomez

tests/rustdoc-ui/doctest/rustflags.rs

@@ -0,0 +1,13 @@
+//@ check-pass
+//@ compile-flags: --test
+//@ rustc-env:RUSTFLAGS=--cfg=testcase_must_be_present
+//@ normalize-stdout-test: "tests/rustdoc-ui/doctest" -> "$$DIR"
+//@ normalize-stdout-test: "finished in \d+\.\d+s" -> "finished in $$TIME"
+
+/// ```
+/// #[cfg(testcase_must_be_present)]
+/// fn must_be_present() {}
+///
+/// fn main() { must_be_present() }
+/// ```
+pub struct Bar;

tests/rustdoc-ui/doctest/rustflags.stdout

@@ -0,0 +1,6 @@
+
+running 1 test
+test $DIR/rustflags.rs - Bar (line 7) ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+