Fix `threads::init() was not called at startup!`

Happens when running `cargo nextest run test_replace_home_directory_with_tilde`, due to process-isolation model.

Backtrace shows:
    EnvStack::set_one(HOME)
    EnvStack::set
    env_dispatch_var_change
    reader_current_data
    reader_data_stack
    assert_is_main_thread
    is_main_thread
    main_thread_id

Fix by using TestEnvironment instead, as this is a unit-test and does
not need anything more.