test: re-sync Manifest.tests.ps1 to canonical template (Module-Dependency variant uplift)

[tablackburn]

Background

tests/Manifest.tests.ps1 in this repo is the older Module-Dependency variant inherited from an earlier version of PowerShellModuleTemplate. It has drifted well behind the current canonical template and should be re-synced.

Spun off from #55 (the test-scaffolding re-sync), which deliberately scoped Manifest.tests.ps1 out so the style sweep stayed reviewable.

Divergence (repo vs. canonical template)

	This repo	Canonical template
Line count	~196	~335
tests/ManifestHelpers.psm1	absent	shipped
Test-VersionConstraint SemVer helper	absent	used (3 call sites)
$isTemplate skip guard	absent	$isTemplate = Test-Path -LiteralPath (Join-Path … 'CHANGELOG.template.md'), applied via -Skip:$isTemplate
Join-Path parameter	-Child (non-canonical abbreviation, line 163)	-ChildPath

Scope

• Pull tests/Manifest.tests.ps1 from the current template (post-fix: Validate PSScriptAnalyzer availability and harden WhatIf tests #36 canonical style: name parameters only on 2+ argument calls).
• Add tests/ManifestHelpers.psm1 (required by the new manifest test for Test-VersionConstraint).
• Fix the -Child → -ChildPath abbreviation.
• Reconcile any genuine local divergences the same way style: re-sync test scaffolding to canonical template (post-#36) #55 did for Help.tests.ps1 (preserve repo-specific bootstrap; adopt canonical everywhere else).

Acceptance

• ./build.ps1 green; manifest tests run (and the changelog-vs-manifest check skips correctly when this is the template)
• CI passes on Linux / macOS / Windows + Windows PowerShell 5.1
• PSScriptAnalyzer clean
• Manifest.tests.ps1 matches canonical template except documented local divergences

Related

• style: re-sync test scaffolding to canonical template (post-#36) #55 — test-scaffolding re-sync (Help / Meta / MetaFixers); explicitly deferred this file
• PowerShellModuleTemplate#36 — the scoped named-parameter rule the sync should follow