spirv-fuzz: Add tests for fuzzer_util.cpp

`fuzzer_util.cpp` has many functions that change SPIR-V module. These include `Add*` functions (e.g. `AddVectorType` and others). It would be good to have some tests for them.

Start with fuzzer_util.cpp and inspect the Add* functions, including AddVectorType, to determine the test cases needed. Done means tests cover the requested module-changing helpers and pass in the project's test run; no test path or command is specified.

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