Run integration tests through the oha binary and simplify test setup

[hatoo]

Integration tests previously called oha::run through a library target, bypassing the executable's entry point. Run the Cargo-built binary via CARGO_BIN_EXE_oha instead, capturing its exit status and output for failure diagnostics. Move the implementation into main.rs and retain argument-parsing checks as unit tests.

The integration tests now use current-thread Tokio runtimes. Simplify argument handling with AsRef<OsStr>, merge the proxy helpers, and parameterize JSON schema validation. Bind TCP and QUIC servers to OS-assigned ports while retaining their sockets, and embed the HTTP/3 test certificates with include_bytes!.

Validation on Linux:

• Integration tests: default (37 passed), HTTP/3 (42 passed), native-tls (37 passed).
• Unit tests: 40 passed after moving the implementation into main.rs.
• cargo fmt --all --check
• cargo clippy --all-targets --all-features -- -D warnings