utilize compiletest_use_stage0_libtest option

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/src/core/build_steps/tool.rs

@@ -425,11 +425,14 @@ macro_rules! bootstrap_tool {
                     }
                 )*
 
+                let is_unstable = false $(|| $unstable)*;
+                let compiletest_wants_stage0 = $tool_name == "compiletest" && builder.config.compiletest_use_stage0_libtest;
+
                 builder.ensure(ToolBuild {
                     compiler: self.compiler,
                     target: self.target,
                     tool: $tool_name,
-                    mode: if false $(|| $unstable)* {
+                    mode: if is_unstable && !compiletest_wants_stage0 {
                         // use in-tree libraries for unstable features
                         Mode::ToolStd
                     } else {