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test: isolate repository fixtures and restore Windows reference coverage #3164

Job	Run time
install/import (macos-latest, 3.8)	1m 16s
install/import (ubuntu-24.04, 3.9)	20s
install/import (windows-latest, 3.9)	1m 6s
install/import (ubuntu-latest, 3.10)	14s
test (ubuntu, 3.13)	5m 22s
install/import (windows-latest, 3.10)	1m 6s
install/import (macos-latest, 3.9)	46s
install/import (windows-latest, 3.8)	1m 1s
install/import (ubuntu-24.04, 3.8)	15s
install/import (macos-latest, 3.10)	39s
test (windows, 3.12)	22m 41s
test (windows, 3.14t)	21m 29s
test (windows, 3.11)	22m 42s
test (ubuntu, 3.12)	5m 14s
test (windows, 3.13)	22m 42s
test (ubuntu, 3.11)	4m 34s
test (ubuntu, 3.15)	3m 26s
test (macos, 3.14)	10m 0s
test (windows, 3.15)	22m 43s
test (windows, 3.14)	18m 57s
test (windows, 3.15t)	21m 25s
test (ubuntu, 3.14t)	3m 35s
test (macos, 3.13)	15m 9s
test (ubuntu, 3.14)	3m 46s
test (macos, 3.12, gix)	6m 32s
test (macos, 3.12)	14m 43s
test (macos, 3.11)	16m 14s
test (ubuntu, 3.12, gix)	6m 43s
test (ubuntu, 3.15t)	5m 41s
test (windows, 3.12, gix)	16m 27s
test (macos, 3.15)	9m 57s
	4h 46m 45s