Add GIX_TEST_FIXTURE_HASH to gix-testtools #7480

Job	Run time
test	11m 31s
test-journey	3m 45s
Check MSRV badge	11s
test-fast (windows-11-arm)	12m 17s
cargo check MSRV (ubuntu-latest)	1m 37s
cargo check MSRV (windows-2025)	3m 0s
test-fast (windows-latest)	8m 0s
pure-rust-build	2m 12s
test-fast (ubuntu-24.04-arm)	2m 27s
check-no-persist-credentials	6s
test-fast (macos-latest)	3m 2s
test-fixtures-windows (windows-11-arm)	17m 20s
check-blocking	8s
cargo-deny-advisories	31s
test-32bit (i386)	6m 3s
test-fast (ubuntu-latest)	2m 42s
test-32bit-windows-size-doc	6m 36s
test-fixtures-windows (windows-latest)	14m 5s
WebAssembly (wasm32-wasip1)	43s
test-32bit (arm32v7)	4m 34s
WebAssembly (wasm32-unknown-unknown)	1m 7s
cargo-deny	28s
lint	4m 4s
Tests pass	4s
	1h 46m 33s

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