tests: Migrate create_slide.list.sh into cargo xtask function. (#2957)

The new xtask function makes the helper code
- more readable
- more reliable due to better error checking
- be in the same place as other helper functions
- and more aligned to the skillset relevant for contributing in this
repository.

The shell script grew and was not readable for everyone anymore without
deeper knowledge.

mitigates #2941 in a more reliable way but does still not fully fix the
root cause

.github/workflows/build.yml

@@ -162,12 +162,7 @@ jobs:
         working-directory: ./tests
       - name: Test Javascript
         if: matrix.language == 'en'
-        run: |
-          ./src/slides/create-slide.list.sh
-          npm test
-        env:
-          TEST_BOOK_DIR: ../book/comprehensive-rust-${{ matrix.language }}/html
-        working-directory: ./tests
+        run: cargo xtask web-tests --dir book/comprehensive-rust-${{ matrix.language }}/html
 
   po-diff:
     name: Translation diff