Plottable: Tests fail for Linux

Output from cargo test:

---- plottable::test::plot_params_fdir stdout ---- thread 'plottable::test::plot_params_fdir' panicked at opossum/src/plottable.rs:1919:14: called Result::unwrap() on an Err value: Other("Parameter of plot argument "FDir(".\\")" is invalid!") note: run with RUST_BACKTRACE=1 environment variable to display a backtrace

---- plottable::test::plot_params_fpathh stdout ---- thread 'plottable::test::plot_params_fpathh' panicked at opossum/src/plottable.rs:1936:9: assertion failed: plt_params.get_fpath().unwrap().ends_with("opossum\opossum\test_name.png")