[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/windows2019.json

@@ -274,8 +274,6 @@
             "environment_vars": [
                 "TOOLSET_JSON_PATH={{user `toolset_json_path`}}"
             ],
-            "elevated_user": "SYSTEM",
-            "elevated_password": "",
             "scripts": [
                 "{{ template_dir }}/scripts/Installers/Install-Msys2.ps1"
             ]