Add tests for --output-to-stdout option

Author: GuillaumeGomez

tests/rustdoc-ui/json-stdout-2.rs

@@ -0,0 +1,8 @@
+// Checks that the `--output-to-stdout` option prints to stdout.
+
+//@ compile-flags: -Zunstable-options --output-to-stdout --output-format=json
+//@ check-pass
+
+#![no_std]
+
+pub struct Foo;

tests/rustdoc-ui/json-stdout-2.stdout

@@ -0,0 +1,5 @@
+// Checks that the `--output-to-stdout` option is unstable.
+
+//@ compile-flags: --output-format=json --output-to-stdout
+
+pub struct Foo;

tests/rustdoc-ui/json-stdout-3.rs

@@ -0,0 +1,2 @@
+error: the `-Z unstable-options` flag must also be passed to enable the flag `output-to-stdout`
+

tests/rustdoc-ui/json-stdout-3.stderr

@@ -0,0 +1,5 @@
+// Checks that the `--output-to-stdout` option is unstable.
+
+//@ compile-flags: --output-format=json --output-to-stdout
+
+pub struct Foo;

tests/rustdoc-ui/json-stdout.rs

@@ -0,0 +1,2 @@
+error: the `-Z unstable-options` flag must also be passed to enable the flag `output-to-stdout`
+