Test disable_urandom_fallback in CI

Author: newpavlov

.github/workflows/tests.yml

@@ -58,6 +58,7 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - run: cargo test
       - run: cargo test --features=std
+      - run: cargo test --features=disable_urandom_fallback
       - run: cargo test --features=custom # custom should do nothing here
       - if: ${{ matrix.toolchain == 'nightly' }}
         run: cargo test --benches