make test_temp_path_resolve_existing_cwd() work with symlink in TMPDIR

Here's a simple way to reproduce a test failure before this commit: ``` ln -s /tmp /tmp/here TMPDIR=/tmp/here cargo test ``` That fails something like this: ``` test test_temp_path_resolve_existing_cwd ... FAILED failures: ---- test_temp_path_resolve_existing_cwd stdout ---- thread 'test_temp_path_resolve_existing_cwd' (137617) panicked at tests/namedtempfile.rs:376:5: assertion `left == right` failed left: "/tmp/.tmpDu1A7l/foo" right: "/tmp/here/.tmpDu1A7l/foo" note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace ``` This commit fixes it by canonicalizing the CWD path in the test.