difftest: fix 5 difftests silently passing on zeroed inputs #1644

Job	Run time
Difftest (macOS-latest)	12m 27s
Difftest (windows-latest)	23m 9s
Lint	5m 32s
Test (macOS-latest)	13m 46s
Test (ubuntu-latest)	12m 5s
Compiletest (ubuntu-latest)	7m 21s
cargo-gpu-os (windows-latest)	14m 30s
cargo-gpu-os (ubuntu-latest)	7m 26s
Test (windows-latest)	22m 17s
cargo-gpu-os (macos-latest)	7m 40s
Difftest (ubuntu-latest)	12m 14s
Compiletest (windows-latest)	17m 13s
cargo-deny	1m 3s
Android (ubuntu-latest, aarch64-linux-android)	8m 49s
Compiletest (macOS-latest)	8m 25s
cargo-gpu-backwards-compat (2326b87fe1542eeb898065e36ac949307b55386d, =0.30.7)	7m 8s
cargo-gpu-backwards-compat (877bd8697a15f3e6d09446a5e1807e6237ca1dac)	7m 17s
cargo-gpu-backwards-compat (e767f24f2565baf1a71bbaf84d453d181cab2417)	6m 56s
cargo-gpu-backwards-compat (f79c4181a5dc2d37303947b113f190930c6c1ce6, =0.30.7)	7m 3s
cargo-gpu-backwards-compat (30896871ba00e668029ccb724f1438202b284708)	7m 6s
cargo-gpu-backwards-compat (0.9.0, 0.24.2)	6m 41s
cargo-gpu-backwards-compat (eea8998df9dc2fd8e7a65c5b5b7ae20c238a665a, 0.29.3)	6m 17s
test_success	3s
	3h 42m 28s

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