CI-windows.yml: also run tests in Windows Server (Core) Docker container #1198

Job	Run time
build (windows-2025, Debug)	55s
build (windows-2025, Release)	1m 27s
test-core (mcr.microsoft.com/windows/server:ltsc2025, Debug)	2m 14s
test-core (mcr.microsoft.com/windows/server:ltsc2025, Release)	2m 13s
test-core (mcr.microsoft.com/windows/servercore:ltsc2025, Debug)	2m 15s
test-core (mcr.microsoft.com/windows/servercore:ltsc2025, Release)	2m 14s
	11m 18s

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