Replace deprecated cargo_bin with cargo_bin! macro

[Copilot]

The assert_cmd::cargo::cargo_bin() function relies on Cargo's internal directory structure to locate test binaries. This breaks when CARGO_BUILD_BUILD_DIR is set, and will fail with upcoming Cargo changes (rust-lang/cargo#16147, rust-lang/cargo#15010).

Changes

Separated integration test utilities from shared test utilities

• Created tests/integration_util.rs containing run() and main_binary() using cargo_bin!() macro
• Removed these functions from tests/util/mod.rs (shared by integration and unit tests)
• Updated tests/main.rs imports

Why the separation?
The cargo_bin!() macro expands to env!("CARGO_BIN_EXE_cargo-mutants"), which is only available at compile-time for integration test binaries. The shared tests/util/mod.rs is included in unit tests via src/main.rs, where this environment variable doesn't exist. Separating the modules allows cargo check --all-targets to succeed while using the stable CARGO_BIN_EXE_* API.

// Before: relies on target dir inference
cargo_bin("cargo-mutants")

// After: uses stable environment variable
cargo_bin!("cargo-mutants")

Original prompt

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This section details on the original issue you should resolve

<issue_title>Tests rely on deprecated assert_cmd::cargo_bin which will break on future Cargo versions</issue_title>
<issue_description>

cargo-mutants/tests/util/mod.rs

Line 27 in 3dd6989

pub static ref MAIN_BINARY: PathBuf = assert_cmd::cargo::cargo_bin("cargo-mutants");

cargo_bin relies on implementation details of cargo that will soon break (rust-lang/cargo#16147, rust-lang/cargo#15010).

You can test this by running:

CARGO_BUILD_BUILD_DIR="{cargo-cache-home}/build/{workspace-path-hash}" cargo test

The replacement is assert_cmd::cargo_bin! </issue_description>

Comments on the Issue (you are @copilot in this section)

@sourcefrog Thanks for letting me know!

• Fixes Tests rely on deprecated assert_cmd::cargo_bin which will break on future Cargo versions #578

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