spirv-fuzz: Add tests for fuzzerutil::Add* functions

There are a bunch of `Add*` functions in the `fuzzerutil` namespace (e.g. `AddIntegerType`, `AddFloatType`, `AddFunctionType`, `AddVariableIdToEntryPointInterfaces`, `AddGlobalVariable`, `AddLocalVariable`, `AddVectorType`, `AddStructType`). They are used by transformations to add various instructions to the module. It would be good to provide unittests for them.

Start by locating the fuzzerutil namespace definitions and any existing unittest conventions. Add unit coverage for the listed Add* functions, including type, interface, global-variable, and local-variable helpers, and verify that the tests exercise the instructions they add to a module.

Written by the indexing model from the issue text.