[Windows] Improve Pester tests coverage for MSYS2 (#3595)

* Add ShouldReturnZeroExitCodeWithParam function and a new set of tests for MSYS2 tools

* Change get-command -> Get-Command

* Remove elevated user, return original path after msys installation and tests, resolve nitpicks

* Add BeforeEach and AfterEach blocks
These blocks change the path before each test
and return the original path after each test

* Change get-command -> Get-Command

images/win/scripts/Installers/Install-Msys2.ps1

@@ -88,6 +88,7 @@ foreach ($arch in $archs)
   pacman.exe -Q | grep ^${arch}-
 }
 
+$env:PATH = $origPath
 Write-Host "`nMSYS2 installation completed"
 
 Invoke-PesterTests -TestFile "MSYS2"