I would recommend you to use an array of parameters and the operator &. See the examples in my answer in here: http://stackoverflow.com/questions/3592851/executing-a-command-stored-in-a-variable-from-powershell/3593445#3593445

In this case the code should be something like this:

$MSTestCall = "C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\MSTest.exe"
$MSTestArguments = @('/resultsFile:out.trx', '/testsettings:C:\someDirectory\local64.testsettings')

foreach($file in Get-ChildItem $CurrentDirectory)  
{ 
    if($file.name -match "\S+test\S?.dll$" ) 
    { 
        $MSTestArguments += "/TestContainer:" + $file
    } 
} 

& $MSTestCall $MSTestArguments