Add tests for symbol file name generation

Fix #732.
Something like this
```fsharp
let [<Literal>] private tempFileName = "tmp"

/// Get file name from symbol's full name and escape illegal characters
let getFileNameFromSymbol (symbol: FSharpSymbol) =    
    let fileName =        
        match symbol with
        | MemberFunctionOrValue mem ->
            mem.LogicalEnclosingEntity.TryGetFullName()
        | UnionCase uc ->
            match uc.ReturnType with
            | TypeWithDefinition entity ->
                entity.TryGetFullName()
            | _ -> None
        | Field (field, _) -> field.DeclaringEntity.TryGetFullName()
        | _ -> Option.attempt(fun _ -> symbol.FullName)
        |> Option.getOrElse tempFileName
            
    let regexSearch = String(Path.GetInvalidFileNameChars()) + String(Path.GetInvalidPathChars())
    let r = Regex(String.Format("[{0}]", Regex.Escape(regexSearch)))
    r.Replace(fileName + ".fsi", "")
```