drop "Unit tests" step in CircleCI workflow

we already run the unit tests in the `jl_test`,
we save 2-3 minutes of CI time by
dropping `ci_prepare.jl` and replacing it
by `Pkg.instantiate()`

Author: etpinard

.circleci/config.yml

@@ -33,14 +33,10 @@ jobs:
             echo "CIRCLE_REPOSITORY_URL: ${CIRCLE_REPOSITORY_URL}"
             echo $CIRCLE_JOB > circlejob.txt
 
-      - run:
-          name: 🔎 Unit tests
-          command: |
-            julia test/ci_prepare.jl
-
       - run:
           name: ⚙️  Integration tests
           command: |
+            julia --project -e 'import Pkg; Pkg.instantiate(); Pkg.update();'
             python -m venv venv
             . venv/bin/activate
             pip install --upgrade pip wheel