Only create a single temporary directory in TestEnv

[niklasmohrin]

Currently, we have

struct TestEnv {
    pub cache_dir: TempDir,
    pub custom_pages_dir: TempDir,
    pub config_dir: TempDir,
    pub default_features: bool,
    pub features: Vec<String>,
}

Instead, there should only be a single TempDir and all files and directories needed should be created inside that one.