Added test package in nix flake to automatically run tests.

To be able to do such, it was required to add the vendor directory
to the git project.

vendor/denopkg.com/chiefbiiko/sha1@master/deps.ts

@@ -0,0 +1,4 @@
+export {
+  encode,
+  decode
+} from "https://denopkg.com/chiefbiiko/std-encoding@v1.1.1/mod.ts";