Refactor ci.sh into workflow steps

Resolves https://github.com/shimming-toolbox/shimming-toolbox/pull/183/files/d70bc78b6039140cd7fc5a9e01b37ce95c6b3816#r532428393
Related to #840

**Why this change was necessary**
Refactoring a multistep CI script into distinct workflow steps gives a
clearer view of where a failure occurred.

**What this change does**
Breaks CI script steps into 2 new steps in the workflow YAML.

**Any side-effects?**
None.

.github/workflows/tests.yml

@@ -13,6 +13,8 @@ on:
 
 jobs:
   test:
+    env:
+      FSLOUTPUTTYPE: NIFTI_GZ
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
@@ -60,15 +62,20 @@ jobs:
           echo "Download prelude for macOS"
           mkdir prelude
           curl -o prelude/prelude -JL https://github.com/shimming-toolbox/binaries/raw/master/prelude_macos
+          sudo install prelude/prelude /usr/local/bin
 
       - name: prelude Ubuntu
         if: contains(matrix.os, 'ubuntu')
         run: |
           echo "Download prelude"
           st_download_data prelude
+          sudo install prelude/prelude /usr/local/bin
 
-      - name: Run CI script
-        run: ./ci.sh
+      - name: Check dependencies
+        run: st_check_dependencies
+
+      - name: Run unit tests
+        run: py.test . -v --cov shimmingtoolbox/ --cov-report term-missing
 
       - name: macOS Shellcheck
         if: contains(matrix.os, 'macos')