test: Add a crust function stress test

src/test/run-pass/crust-stress.rs

+// This creates a bunch of yielding tasks that run concurrently
+// while holding onto C stacks
+
+native mod rustrt {
+    fn rust_dbg_call(cb: *u8,
+                     data: ctypes::uintptr_t) -> ctypes::uintptr_t;
+}
+
+crust fn cb(data: ctypes::uintptr_t) -> ctypes::uintptr_t {
+    if data == 1u {
+        data
+    } else {
+        task::yield();
+        count(data - 1u) + count(data - 1u)
+    }
+}
+
+fn count(n: uint) -> uint {
+    rustrt::rust_dbg_call(cb, n)
+}
+
+fn main() {
+    iter::repeat(100u) {||
+        task::spawn {||
+            count(5u);
+        };
+    }
+}
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