Validator test: artificially lower limit on local var limit

We need this to reduce the test time on Visual Studio debug builds.
AppVeyor times out at 300s for a process.

Artificially lower the local variable limit check for testing purposes.
We don't get much value from using the full 500K+ variables.
Use a limit of 5000 instead.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/1632