Core.Tests: add NoPartialFunctions regression test

Added regression test for NoPartialFunctions for case when
`Value` instance method is called on non value that is not
Option, but another Option value exists in scope.

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

@@ -62,6 +62,19 @@ module Program =
         Assert.IsTrue(this.ErrorExistsAt(7, 34))
         this.AssertErrorWithMessageExists("Consider using pattern matching instead of partial function/method 'Option.Value'.")
 
+    [<Test>]
+    member this.``No error for calling Value on ref type (regression)``() =
+        this.Parse("""
+namespace Foo
+module Program =
+    let foo = None
+    let bar = ref 0
+
+    let printFoo() =
+        System.Console.WriteLine (bar.Value.ToString())""")
+
+        Assert.IsTrue this.NoErrorsExist
+
     [<Test>]
     member this.``Error for Option.Value (List.tryHead test case)``() =
         this.Parse("""