appveyor: Add -TestPath param to test.ps1

Author: r15ch13

test/bin/test.ps1

@@ -2,12 +2,15 @@
 #requires -Modules @{ ModuleName = 'BuildHelpers'; ModuleVersion = '2.0.1' }
 #requires -Modules @{ ModuleName = 'Pester'; ModuleVersion = '4.4.0' }
 #requires -Modules @{ ModuleName = 'PSScriptAnalyzer'; ModuleVersion = '1.17.1' }
+param(
+    [String] $TestPath = 'test/',
+)
 
 $resultsXml = "$PSScriptRoot/TestResults.xml"
 $excludes = @()
 
 $splat = @{
-    Path = 'test/'
+    Path = $TestPath
     OutputFile = $resultsXml
     OutputFormat = 'NUnitXML'
     PassThru = $true