Add a test for const variable initialization

This test tests that built-in functions, operators and constructors
are allowed in constant expressions used to initialize const variables.

The test does not cover what values the constant expressions return, or
all input value types of all built-in functions, but every built-in
function is tested with at least one set values.
Merge pull request #801 from NVIDIA/constant-expression-test

Add a test for const variable initialization