Fixed the bug "Unable to find type [PSPlot]"

Also added a test for it in ImportExcel.Tests.ps1

The previous fix was broke again by commit:
bd40cfe829 (diff-f50d26003c0baf98f417a44fc1cac92b)

ImportExcel.psm1

@@ -39,7 +39,6 @@ if ($PSVersionTable.PSVersion.Major -ge 5) {
     . $PSScriptRoot\Plot.ps1
 
     Function New-Plot {
-        [OutputType([PSPlot])]
         Param()
 
         [PSPlot]::new()