test: fix DateTimeImmutable plus one year calls

[phil-davis]

Some calls were:

new \DateTimeImmutable(date('Y', strtotime('+1 year')))

With just the 'Y' format it gets '2027'. and date('2027') returns 20:27 of today. And so if the test is run before 20:27 then the date is in the future, and the test passes. But if the test is run after 20:27 and before midnight, it will fail because the expiration date requested is in the past.

Actually we can pass '+1 year' directly to DateTimeImmutable and avoid messing about with a date format in the middle. So I changed all '+1 year' usages in tests to do that.

[kw-security]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Code Security	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.

[phil-davis]

This should fix the “intermittent” test failures like in #306 last night.