Add a Pester test suite for the collectors

84 tests in collectors/tests, run with Invoke-CollectorTests.ps1 (~15 s,
PowerShell 7.2+ and Pester 5.5+):

- Module.Tests: the shared module (property lookup, time format,
  config/lookback, sent-run state and pruning, dry runs, exit codes,
  401 detection)
- Veeam/Mars/Wsb/Vb365.Tests: each collector run in its own process
  against its mock and a stand-in ingest API, asserting the behaviours
  confirmed on real servers: VBR 13 live schedule, copy jobs, MARS UTC
  times and ErrorCode 0, WSB culture-formatted times and nested VMs,
  VB365 organization mapping, log prefixes and retries, state and
  resending, rejected keys
- Installers.Tests: config, files and scheduled task from each
  installer, with stand-ins for icacls / Task Scheduler / the key prompt

Collectors run under the test runner's PowerShell, or another shell via
-CollectorShell (e.g. powershell.exe, to test under Windows PowerShell
5.1 like the MARS, WSB and VB365 servers). The mocks take their "now"
from MOCK_NOW so expectations can't drift with the clock; the WSB mock
gains -RunningJobFinished for the running-to-finished case.

Each of 12 past bugs, put back one at a time, fails the test for its
behaviour (and the runner exits 1).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

collectors/vb365/tests/MockVb365.ps1

@@ -8,7 +8,8 @@
 #   ./Send-Vb365BackupStatus.ps1 -ConfigPath ./my-test-config.json -DryRun
 
 # Fixed to the top of the hour, and IDs derived from names/times, so repeated runs see the same data
-$now = Get-Date -Minute 0 -Second 0 -Millisecond 0
+# MOCK_NOW (ISO 8601, set by the test suite) pins the time everything is relative to
+$now = if ($env:MOCK_NOW) { [datetime]$env:MOCK_NOW } else { Get-Date -Minute 0 -Second 0 -Millisecond 0 }
 function New-StableGuid([string]$Seed) {
     [guid]::new([Security.Cryptography.MD5]::Create().ComputeHash([Text.Encoding]::UTF8.GetBytes($Seed)))
 }