spirv-fuzz: Enhance test to improve lines covered (#4289)

Fixes #4287 by enhancing the test to get 100% of lines covered.
diff --git a/test/fuzz/transformation_add_constant_scalar_test.cpp b/test/fuzz/transformation_add_constant_scalar_test.cpp
index 91339db..00c0541 100644
--- a/test/fuzz/transformation_add_constant_scalar_test.cpp
+++ b/test/fuzz/transformation_add_constant_scalar_test.cpp
@@ -114,6 +114,11 @@
   transformation = TransformationAddConstantScalar(19, 5, {0, 1, 2}, false);
   ASSERT_FALSE(
       transformation.IsApplicable(context.get(), transformation_context));
+
+  // Tests |words| having 2 words for a 32-bit float type.
+  transformation = TransformationAddConstantScalar(19, 4, {0, 1}, false);
+  ASSERT_FALSE(
+      transformation.IsApplicable(context.get(), transformation_context));
 }
 
 TEST(TransformationAddConstantScalarTest, Apply) {