tests: move to t, allow `yath test` from root

By moving the tests subdirectory to t, we gain the ability to run yath test with no arguments from inside nix develop in the root of the the repo.

(nix develop is necessary in order to set the proper env vars for yath to find our test libraries.)

Created by  Cole Helbling  on March 5, 2021
2JJP76737U2JWJWQ6UDFEAQCGWRAQH46HC6OCIKWMB5QYRXF6DQQC
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