ci: Remove inner quotes from shell variable

shellcheck emits:

 SC2027: The surrounding quotes actually unquote this. Remove or escape them.

Remove inner quotes from shell variable so the feature is correctly printed.

contrib/test.sh

@@ -80,7 +80,7 @@ fi
 # Test each feature
 for feature in ${FEATURES}
 do
-    echo "********* Testing "$feature" *************"
+    echo "********* Testing $feature *************"
     cargo test --verbose --features="$feature"
 done