Support x check rustdoc-gui-test

This is useful for ensuring that changes to compiletest haven't broken
rustdoc-gui-test.

Author: Zalathar

src/bootstrap/src/core/build_steps/check.rs

@@ -821,6 +821,15 @@ tool_check_step!(Compiletest {
     default: false,
 });
 
+// As with compiletest, rustdoc-gui-test is automatically built when running
+// relevant tests. So being able to check it is mainly useful for people
+// working on on rustdoc-gui-test itself, or on its compiletest dependency.
+tool_check_step!(RustdocGuiTest {
+    path: "src/tools/rustdoc-gui-test",
+    mode: Mode::ToolBootstrap,
+    default: false,
+});
+
 tool_check_step!(Linkchecker {
     path: "src/tools/linkchecker",
     mode: Mode::ToolBootstrap,

src/bootstrap/src/core/builder/mod.rs

@@ -1071,6 +1071,7 @@ impl<'a> Builder<'a> {
                 check::Bootstrap,
                 check::RunMakeSupport,
                 check::Compiletest,
+                check::RustdocGuiTest,
                 check::FeaturesStatusDump,
                 check::CoverageDump,
                 check::Linkchecker,