Replace tempdir with std::env::temp_dir()

[alexanderwiederin]

tempdir is an unmaintained crate that has not been updated in eight years. It is used in two places, both in tests:

• tests/test.rs
• src/state/chainstate.rs

This will allow us to remove the tempdir = ">=0.3.6" minimum version constraint that was added in #150.

[alexanderwiederin]

To fully replicate the behavior of tempdir we should also have a mechanism to clean up the directories after the tests have run.