Fix flaky test

src/test/run-make-fulldeps/rustdoc-test-builder/Makefile

@@ -0,0 +1,10 @@
+-include ../../run-make-fulldeps/tools.mk
+
+# How to manually run this
+# $ ./x.py test src/test/run-make/rustdoc-test-builder
+
+all:
+	# Rustc isn't available in UI tests because the only compiler available is the
+	# one built from source, which isn't in PATH.  This needs to be a run-make
+	# test so rustc is available.
+	$(RUSTDOC) --test -Z unstable-options --test-builder $(RUSTC_ORIGINAL) doctest.rs

src/test/run-make-fulldeps/rustdoc-test-builder/doctest.rs

@@ -0,0 +1,4 @@
+/// ```no_run
+/// // This tests that `--test-builder` is accepted as a flag by rustdoc.
+/// ```
+pub struct Foo;