Tests in tests/lib.rs pollute each other, may cause spurious failure.

[kennytm]

Generally, Rust runs tests in parallel.

In https://github.com/aesir-vanir/vidar/blob/master/tests/lib.rs, the tests all operate on the same directory structure, and thus a file may be overridden by other tests, for instance

Thread 1	Thread 2
Test no_file()
	Test invalid_kind()
setup(Some(most))
Creates all files except int.env
	setup(None)
	Creates int.env as well
Check Environment::try_from("int")
Exists, returns Ok
Thus assert!(false)
💥

Each test should run inside their own temporary directory, instead of reusing the same env::temp_dir().