Replace the absolute paths to test files with the relative paths

[Pavel-Irzhauski]

As long as the absolute path env!("CARGO_MANIFEST_DIR") is used to access the test files it is often impossible to test a cross-platform build on the target platform since the corresponding path can't be created. On the other hand according to The Cargo Book

The working directory when running each unit and integration test is set to the root directory of the package the test belongs to. Setting the working directory of tests to the package's root directory makes it possible for tests to reliably access the package's files using relative paths, regardless from where cargo test was executed from.