test(zstd): Fix 'zstd' extraction error in test

[niheaven]

Fix Appveyor error caused by wrong zstd.exe extraction.

Description

• Update zstd to v1.5.1 and change its 7z extraction param (This is why CI cannot find zstd)
• Fix Mock function in zstd test cases (This is why nested zstd test fails)
• Some unrelated tweaks
  • Install helper and set EnvVars on demand (Move from init.ps1 to test.ps1)
  • Move helper EnvVars from appveyor.yml to test.ps1
  • Change condition of Mock Get-AppFilePath to CI_WINDOWS so developers could simulate test in CI by setting $env:CI_WINDOWS = $true and modifying the helper downloading part of test.ps1 file

Motivation and Context

Relates to #4608 (comment)

How Has This Been Tested?

https://ci.appveyor.com/project/niheaven/scoop/builds/42177522/job/l8b7kyla9deyiohx

Checklist:

• I have read the Contributing Guide.
• I have updated the documentation accordingly.
• I have updated the tests accordingly.

[niheaven]

GitHub recommends _PATH suffix that point to a location on the filesystem (https://docs.github.com/en/actions/learn-github-actions/environment-variables#naming-conventions-for-environment-variables), so modify lessmsi, innounp and zstd environment variable names.

[chawyehsu]

GitHub recommends _PATH suffix that point to a location on the filesystem (https://docs.github.com/en/actions/learn-github-actions/environment-variables#naming-conventions-for-environment-variables), so modify lessmsi, innounp and zstd environment variable names.

How about adding a SCOOP_ prefix for them.

[niheaven]

How about adding a SCOOP_ prefix for them.

Just likes SCOOP_HELPERS_PATH? i.e. SCOOP_LESSMSI_PATH, SCOOP_INNOUNP_PATH and SCOOP_ZSTD_PATH?

[chawyehsu]

Yes

[niheaven]

Okay, I'll add them. Done.