Add regression test for enable to run compiler in doctest

Author: GuillaumeGomez

tests/run-make/rustdoc-test-builder/foo.rs

@@ -0,0 +1,3 @@
+//! ```
+//! let x = 12;
+//! ```

tests/run-make/rustdoc-test-builder/rmake.rs

@@ -0,0 +1,22 @@
+// This test ensures that if the rustdoc test binary is not executable, it will
+// gracefully fail and not panic.
+
+//@ needs-target-std
+
+use run_make_support::{path, rfs, rustdoc};
+
+fn main() {
+    let absolute_path = path("foo.rs").canonicalize().expect("failed to get absolute path");
+    let output = rustdoc()
+        .input("foo.rs")
+        .arg("--test")
+        .arg("-Zunstable-options")
+        .arg("--test-builder")
+        .arg(&absolute_path)
+        .run_fail();
+
+    output.assert_stdout_contains("Failed to run compiler: ");
+    output.assert_stderr_not_contains("the compiler unexpectedly panicked. this is a bug.");
+    // Just in case...
+    output.assert_stdout_not_contains("the compiler unexpectedly panicked. this is a bug.");
+}