Fix flaky test

Rustdoc writes to a pipe, and if the program on the other side has
exited, it will exit with SIGPIPE. To avoid that happening, call `rustc`
instead of `true`, which will wait to exit until it sees EOF on stdin.

- Use a run-make-fulldeps test so that rustc is available in CI

  This is not exactly right, because the test shouldn't require building
  rustc twice, but it avoids having to build rustdoc for run-make tests.

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;