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>

pkg/universe.dagger.io/bats_helpers.bash

@@ -0,0 +1,25 @@
+common_setup() {
+    load "$(dirname "${BASH_SOURCE[0]}")/node_modules/bats-support/load.bash"
+    load "$(dirname "${BASH_SOURCE[0]}")/node_modules/bats-assert/load.bash"
+
+    # Dagger Binary
+    # FIXME: `command -v` must be wrapped in a sub-bash,
+    #   otherwise infinite recursion when DAGGER_BINARY is not set.
+    export DAGGER="${DAGGER_BINARY:-$(bash -c 'command -v dagger')}"
+
+    # Force Europa mode
+    DAGGER_EUROPA="1"
+    export DAGGER_EUROPA
+
+    # Force plain printing for error reporting
+    DAGGER_LOG_FORMAT="plain"
+    export DAGGER_LOG_FORMAT
+
+    # cd into the directory containing the bats file
+    cd "$BATS_TEST_DIRNAME" || exit 1
+}
+
+# dagger helper to execute the right binary
+dagger() {
+    "${DAGGER}" "$@"
+}