Fix UseXdp parameter type in test.ps1 (switch -> string)

test.ps1 declared -UseXdp as [switch] with a string default of "", which
fails PowerShell argument transformation on every invocation that does not
pass -UseXdp, and mis-binds -UseXdp <version> as a positional argument when
it does (landing on -IsolationMode). UseXdp carries an XDP version string
(e.g. 'xdp-v1.1'), so declare it as [string] to match its usage and the
other scripts (prepare-machine.ps1, spin.ps1, recvfuzz.ps1).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: guhetier

scripts/test.ps1

@@ -185,7 +185,7 @@ param (
     [switch]$XdpMapMode = $false,
 
     [Parameter(Mandatory = $false)]
-    [switch]$UseXdp = "",
+    [string]$UseXdp = "",
 
     [Parameter(Mandatory = $false)]
     [switch]$UseQtip = $false,