Add test for NumBuffer compilation error

Author: GuillaumeGomez

tests/run-make/num-fmt/main.rs

@@ -0,0 +1,9 @@
+#![feature(int_format_into)]
+
+use std::fmt::NumBuffer;
+
+fn main() {
+    let x = 0u32;
+    let mut buf = NumBuffer::<u8>::new();
+    x.format_into(&mut buf); //~ ERROR
+}

tests/run-make/num-fmt/rmake.rs

@@ -0,0 +1,21 @@
+// Test to ensure that if you use a `NumBuffer` with a too small integer, it
+// will fail at compilation time.
+
+// FIXME: This test should be a `ui` test, but not possible for the following reasons:
+// * With the default `--emit`, the compilation succeeds, we can go around by using `--emit=link`
+//   but then we get the second problem.
+// * Compilation error is not generated in the JSON output.
+//
+// So for not we go around this problem by moving it in `run-make` until we figure out what's
+// going on.
+
+//@ needs-target-std
+
+use run_make_support::rustc;
+
+fn main() {
+    rustc()
+        .input("main.rs")
+        .run_fail()
+        .assert_stderr_contains("buffer is not big enough");
+}