DirectoryMock.Tests.ps1

$here = Split-Path -Parent $MyInvocation.MyCommand.Path
$sut = (Split-Path -Leaf $MyInvocation.MyCommand.Path) -replace '\.Tests\.', '.' 
. "$here\$sut"

$mockReply = @("Monday.txt", "Tuesday.txt", "Wednesday.txt")

Describe "HasEntryInDirectory" {
    Context "If the filename exists in the directory" {

        Mock Get-ChildItem {return $mockReply}
        $result = HasEntryInDirectory "Monday.txt" "C:\LogEntries\2017\23\"
        It "Returns true" {
            $result | Should Be $true
        }
    }

    Context "If the filename does not exists in the directory" {
        Mock Get-ChildItem {return @("Monday.txt", "Tuesday.txt", "Wednesday.txt")}
        $result = HasEntryInDirectory "Thursday.txt" "C:\LogEntries\2017\23\"

        It "Returns false" {
            $result | Should Be $false
        }
    }
}