test: add `online_tests` feature to control Internet use #673

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Job	Run time
pre-commit	3m 0s
Cargo Machete	1m 1s
determine changes	8s
build binary \| linux x86_64	2m 37s
build binary \| windows aarch64	4m 14s
build binary \| macos aarch64	3m 23s
build binary \| macos x86_64	0s
build binary \| windows x86_64	4m 16s
cargo test \| ubuntu	4m 48s
cargo test \| macos aarch64	4m 36s
cargo test \| macos x86_64	0s
cargo test \| windows	6m 55s
cargo rustdoc \| ubuntu	4m 1s
cargo fmt \| ubuntu	14s
cargo clippy \| ubuntu	57s
Pytest \| linux x86_64	1m 4s
Integration tests \| linux x86_64	0s
Downstream tests \| linux x86_64	0s
Wheel Tests \| linux x86_64 / Test Installation of Common Wheels \| linux-x86_64	1m 59s
Pytest \| macos aarch64	1m 16s
Integration tests \| macos aarch64	0s
Downstream tests \| macos aarch64	0s
Wheel Tests \| macos aarch64 / Test Installation of Common Wheels \| macos-aarch64	1m 17s
Pytest \| windows x86_64	5m 32s
Integration tests \| windows x86_64	0s
Downstream tests \| windows x86_64	0s
Wheel Tests \| windows x86_64 / Test Installation of Common Wheels \| windows-x86_64	3m 26s
	54m 44s

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