Add a generic file watcher function in HelpersCommon.psm1 (#4775)

Add a generic file watcher function in HelpersCommon.psm1 and update some tests to use the `Wait-FileToBePresent`.

test/powershell/Modules/Microsoft.PowerShell.Management/FileSystem.Tests.ps1

@@ -233,12 +233,7 @@ Describe "Basic FileSystem Provider Tests" -Tags "CI" {
             "Test timeout waiting for $cmdLine" | Set-Content -Path $errFile
 
             runas.exe /trustlevel:0x20000 "$powershell -nop -c try { $cmdline -ErrorAction Stop } catch { `$_.FullyQualifiedErrorId | Out-File $errFile }; New-Item -Type File -Path $doneFile"
-
-            $startTime = Get-Date
-            while (((Get-Date) - $startTime).TotalSeconds -lt 15 -and -not (Test-Path $doneFile))
-            {
-                Start-Sleep -Milliseconds 100
-            }
+            Wait-FileToBePresent -File $doneFile -TimeoutInSeconds 15 -IntervalInMilliseconds 100
 
             $err = Get-Content $errFile
             $err | Should Be $expectedError

test/powershell/Modules/Microsoft.PowerShell.Management/Start-Process.Tests.ps1

@@ -133,11 +133,9 @@ Describe "Start-Process tests requiring admin" -Tags "Feature","RequireAdminOnWi
         $fooFile = Join-Path $TestDrive "FooTest.foo"
         New-Item $fooFile -ItemType File -Force
         Start-Process $fooFile
-        $startTime = Get-Date
-        while (((Get-Date) - $startTime).TotalSeconds -lt 10 -and (!(Test-Path $testdrive\foo.txt)))
-        {
-            Start-Sleep -Milliseconds 100
-        }
+
+        Wait-FileToBePresent -File "$testdrive\foo.txt" -TimeoutInSeconds 10 -IntervalInMilliseconds 100
+
         "$testdrive\foo.txt" | Should Exist
         Get-Content $testdrive\foo.txt | Should BeExactly $fooFile
     }