fix(backend): move the cursor when first diff is on second cell

Both termion and crossterm backends were not moving the cursor if the first diff to draw was on the
second cell. The condition triggering the cursor move has been updated to fix this. In addition, two
tests have been added to avoid future regressions.

.github/workflows/ci.yml

@@ -47,6 +47,8 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: test
+        env:
+          RUST_BACKTRACE: full
       - name: "Clippy"
         uses: actions-rs/cargo@v1
         with:
@@ -75,3 +77,5 @@ jobs:
         with:
           command: test
           args: --no-default-features --features=crossterm --tests --examples
+        env:
+          RUST_BACKTRACE: full