test: avoid infinite loop in out-of-stack.rs

This fixes #13238. It avoids an infinite loop when compiling
the tests with `-g`. Without this change, the debuginfo on
`black_box` prevents the method from being inlined, which
allows llvm to convert `silent_recurse` into a tail-call. This
then loops forever instead of consuming all the stack like it
is supposed to. This patch forces inlining `black_box`, which
triggers the right error.

Author: erickt

src/test/run-pass/out-of-stack.rs

@@ -17,6 +17,9 @@ use std::os;
 use std::str;
 
 // lifted from the test module
+// Inlining to avoid llvm turning the recursive functions into tail calls,
+// which doesn't consume stack.
+#[inline(always)]
 pub fn black_box<T>(dummy: T) { unsafe { asm!("" : : "r"(&dummy)) } }
 
 fn silent_recurse() {