ci: run tests with cargo nextest

[lexoliu]

Migrates the CI test runs from cargo test to cargo nextest run (installed via taiki-e/install-action@nextest), matching the setup already used by waterui, gtk-backend, dew and helios.

• cargo test invocations become cargo nextest run with the same package/feature flags.
• Where the old command ran doctests implicitly, a cargo test --doc step keeps them.
• cargo llvm-cov coverage jobs become cargo llvm-cov nextest.
• Test suites with harness = false, wasm-pack, and device/simulator flows are left untouched where nextest cannot enumerate or execute them.