Reorganize some engine tests to make it clean (#4551)

1. The individual test files (.ps1) in `/test/powershell/engine` are moved to the corresponding subfolders.
2. The test asset file `test/powershell/Common/TestTypeFile.ps1xml` is moved to `test/powershell/Common/engine/Api/assets/` so that it's next to the test that is using the file.

test/powershell/engine/Api/InitialSessionState.Tests.ps1

@@ -65,7 +65,7 @@ Describe "TypeTable duplicate types in reused runspace InitialSessionState TypeT
 
         BeforeAll {
 
-            $typeTable = [System.Management.Automation.Runspaces.TypeTable]::new([string[]](Join-Path $PSScriptRoot "../Common/TestTypeFile.ps1xml"))
+            $typeTable = [System.Management.Automation.Runspaces.TypeTable]::new([string[]](Join-Path $PSScriptRoot "assets/TestTypeFile.ps1xml"))
             [initialsessionstate] $iss = [initialsessionstate]::Create()
             $iss.Types.Add($typeTable)
             [runspace] $rs1 = [runspacefactory]::CreateRunspace($iss)
@@ -98,7 +98,7 @@ Describe "TypeTable duplicate types in reused runspace InitialSessionState TypeT
         BeforeAll {
 
             # Create default ISS and add shared TypeTable.
-            $typeTable = [System.Management.Automation.Runspaces.TypeTable]::new([string[]](Join-Path $PSScriptRoot "../Common/TestTypeFile.ps1xml"))
+            $typeTable = [System.Management.Automation.Runspaces.TypeTable]::new([string[]](Join-Path $PSScriptRoot "assets/TestTypeFile.ps1xml"))
             [initialsessionstate] $iss = [initialsessionstate]::CreateDefault2()
             $iss.Types.Add($typeTable)
             $iss.ThrowOnRunspaceOpenError = $true