tests: Basic test infrastructure for Europa Universe

- Each package can include its own bats file (e.g. `universe.dagger.io/yarn/test/yarn.bats`)
- universe.dagger.io includes a common bash helper file
- bats is installed/launched through yarn with minimal setup
- shellcheck is done across the entire repo
- Integrated into our CI

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

tests/tasks/source/test.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-echo -n hello world > /test.txt
+printf "hello world" > /test.txt