Core.Tests: add failing test for UsedUnderscorePrefixedElements

Added failing test for UsedUnderscorePrefixedElements rule when
variable with name beginning with _ is used, but not
immediately after declaration.

tests/FSharpLint.Core.Tests/Rules/Conventions/UsedUnderscorePrefixedElements.fs

@@ -19,6 +19,28 @@ module MyModule =
 
         Assert.IsTrue this.ErrorsExist
 
+    [<Test>]
+    member this.``Use variable with underscore prefix but not immediately after declaration``() =
+        this.Parse """
+module MyModule =
+    let MyFunc () =
+        let _random = System.Random()
+        ()
+        ()
+        _random"""
+
+        Assert.IsTrue this.ErrorsExist
+
+    [<Test>]
+    member this.``Use variable with underscore prefix inside some expression``() =
+        this.Parse """
+module MyModule =
+    let MyFunc () =
+        let _random = System.Random()
+        "someString" + _random.ToString()"""
+
+        Assert.IsTrue this.ErrorsExist
+
     [<Test>]
     member this.``Not using variable with underscore prefix``() =
         this.Parse """