Add conformance-tests crate, move fixtures, and update test paths

[bladecoder]

Motivation

• Centralize conformance fixtures and tests in a dedicated crate so test assets are resolved consistently across workspace members.
• Ensure workspace package names and directories match so Cargo resolves the test crate correctly.
• Remove brittle workspace-relative path fallbacks in integration tests by resolving fixtures relative to the crate manifest.

Description

• Added a new crate conformance-tests and updated the workspace Cargo.toml to include conformance-tests as a member.
• Moved the inkfiles fixtures and most test files from the previous locations into conformance-tests/inkfiles and conformance-tests/tests respectively, and added conformance-tests/Cargo.toml.
• Updated cli-player integration tests to construct fixture paths using Path::new(env!("CARGO_MANIFEST_DIR")).join("../conformance-tests/inkfiles/...") instead of workspace-relative fallbacks.
• Updated test helper get_json_string to use Path::new(env!("CARGO_MANIFEST_DIR")).join(filename) so library tests read fixtures relative to the manifest.

Testing

• Ran cargo test for the workspace; the full test run completed successfully with all unit and integration tests passing.
• cli-player integration tests (basic_tests and test_the_intercept) passed after updating fixture paths.
• Library unit tests and the moved conformance-tests suites all passed in the automated test run.
• Test run emitted non-critical warnings (unused parentheses and deprecated cargo_bin usage) but no test failures.

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Codex Task