add cfail test: borrow can't escape critical sections

tests/cfail/critical-section.rs

+#![deny(warnings)]
+#![feature(const_fn)]
+#![feature(proc_macro)]
+#![no_std]
+
+#[macro_use(task)]
+extern crate cortex_m_rtfm as rtfm;
+extern crate stm32f103xx;
+
+use rtfm::{app, Resource, Threshold};
+
+app! {
+    device: stm32f103xx,
+
+    resources: {
+        static STATE: bool = false;
+    },
+
+    idle: {
+        resources: [STATE],
+    },
+
+    tasks: {
+        EXTI0: {
+            enabled: true,
+            priority: 1,
+            resources: [STATE],
+        },
+    },
+}
+
+fn init(_p: init::Peripherals, _r: init::Resources) {}
+
+fn idle(t: &mut Threshold, r: idle::Resources) -> ! {
+    let state = rtfm::atomic(|cs| {
+        // ERROR borrow can't escape this *global* critical section
+        r.STATE.borrow(cs) //~ error cannot infer an appropriate lifetime
+    });
+
+    let state = r.STATE.claim(t, |state, _t| {
+        // ERROR borrow can't escape this critical section
+        state //~ error cannot infer an appropriate lifetime
+    });
+
+    loop {}
+}
+
+task!(EXTI0, exti0);
+
+fn exti0(_t: &mut Threshold, _r: EXTI0::Resources) {}