doctests: fix merging on stable

Fixes rust-lang#137898

The generated multi-test harness relies on nightly-only APIs,
so the only way to run it on stable is to enable them. Since
tests that use crate attrs don't be merged, there's no way to use
nightly-only features on it anyway.

(cherry picked from commit 5d6eeea)

Author: notriddle

src/librustdoc/doctest.rs

@@ -550,7 +550,12 @@ fn run_test(
     }
 
     compiler.arg("--edition").arg(doctest.edition.to_string());
-    if !doctest.is_multiple_tests {
+    if doctest.is_multiple_tests {
+        // The merged test harness uses the `test` crate, so we need to actually allow it.
+        // This will not expose nightly features on stable, because crate attrs disable
+        // merging, and `#![feature]` is required to be a crate attr.
+        compiler.env("RUSTC_BOOTSTRAP", "1");
+    } else {
         // Setting these environment variables is unneeded if this is a merged doctest.
         compiler.env("UNSTABLE_RUSTDOC_TEST_PATH", &doctest.test_opts.path);
         compiler.env(