test: Stop skipping the signing tests on Windows PowerShell 5.1

[tablackburn]

Closes #197

The defect

Twenty-two tests across the three signing test files were guarded with -Skip:(-not $IsWindows). $IsWindows does not exist on Windows PowerShell 5.1, so it is $null, -not $null is $true, and every one of them skipped — on Windows, on the engine where store-based code signing is most common.

Measured under powershell.exe 5.1 before the change:

File	passed	skipped
Get-PSBuildCertificate.tests.ps1	8	18
Invoke-PSBuildModuleSigning.tests.ps1	9	2
New-PSBuildFileCatalog.tests.ps1	8	2

After:

File	passed	skipped
Get-PSBuildCertificate.tests.ps1	26	0
Invoke-PSBuildModuleSigning.tests.ps1	11	0
New-PSBuildFileCatalog.tests.ps1	10	0

So #194's SkipCertificateValidation fallback had never once been exercised on 5.1 in CI.

The fix

The guard is now the shape the product code already uses — twelve lines from one of these call sites:

# $IsWindows does not exist on Windows PowerShell 5.1 (Desktop edition), where it is $null
# and the platform is always Windows; only treat the platform as non-Windows when $IsWindows
# is explicitly $false (PowerShell 7+ on Linux/macOS).
if ($null -ne $IsWindows -and -not $IsWindows) {

"Windows-only" meant platform, not edition — the comment justifying the guard said exactly that while the guard itself did not. That comment now also records why the shape matters, so the next person does not reintroduce the short form.

The risk that didn't materialise

This was filed rather than fixed inline because the mocks might have behaved differently on the older engine — Mock -ModuleName PowerShellBuild -CommandName Get-ChildItem with the -CodeSigningCert dynamic parameter — so un-skipping could have surfaced failures in the tests rather than the code.

It didn't. All twenty-two pass on 5.1 unchanged, no test bodies touched.

Wider than reported

The issue described this as the convention within one file. It was three — the other two signing test files carried the same inverted guard and are corrected here. grep -rn '\-not \$IsWindows' tests/ now returns nothing outside the corrected form.

Worth noting the repo already had the right pattern elsewhere: build.tests.ps1 and Build-PSBuildHelp.tests.ps1 both use $IsWindows -or $null -eq $IsWindows. Only the signing tests got it inverted.

Verification

• PowerShell 7 full suite: 503 passed, 0 failed, 3 skipped; Analyze clean.
• Windows PowerShell 5.1: the three files run 47 tests, 0 failed, 0 skipped.
• All three files verified CRLF-only, no lone LF, no \r\r\n.

🤖 Generated with Claude Code

https://claude.ai/code/session_01U1Jhu7fgTRJq7LK5MuKteE

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Test Results

    4 files  ±0    785 suites  ±0   2m 26s ⏱️ +12s
  508 tests ±0    505 ✅ ± 0   3 💤 ± 0  0 ❌ ±0 
2 015 runs  ±0  1 943 ✅ +22  72 💤  - 22  0 ❌ ±0 

Results for commit 3bdb642. ± Comparison against base commit ff45f77.