tests: add system tests for inverted span issue

Author: calebcartwright

Diff for: tests/source/issue_4374.rs

@@ -0,0 +1,13 @@
+fn a<F>(_f: F) -> ()
+where
+  F: FnOnce() -> (),
+{
+}
+fn main() {
+  a(|| {
+    #[allow(irrefutable_let_patterns)]
+    while let _ = 0 {
+      break;
+    }
+  });
+}

Diff for: tests/target/issue_3934.rs

@@ -0,0 +1,8 @@
+mod repro {
+    pub fn push() -> Result<(), ()> {
+        self.api.map_api_result(|api| {
+            #[allow(deprecated)]
+            match api.apply_extrinsic_before_version_4_with_context()? {}
+        })
+    }
+}

Diff for: tests/target/issue_4374.rs

@@ -0,0 +1,13 @@
+fn a<F>(_f: F) -> ()
+where
+    F: FnOnce() -> (),
+{
+}
+fn main() {
+    a(|| {
+        #[allow(irrefutable_let_patterns)]
+        while let _ = 0 {
+            break;
+        }
+    });
+}