Modify integration tests to fail on panic

tests/fixture/success/compSuccess.rs

@@ -0,0 +1,2 @@
+fn main() {
+}

tests/fixture/success/info.toml

@@ -0,0 +1,7 @@
+[[exercises]]
+path = "compSuccess.rs"
+mode = "compile"
+
+[[exercises]]
+path = "testSuccess.rs"
+mode = "test"

tests/fixture/success/testSuccess.rs

@@ -0,0 +1,4 @@
+#[test]
+fn passing() {
+    assert!(true);
+}