Test explicit initializers of program scope variables

There is currently no test that program scope variables that have explicit initializers are correctly initialized with the stated value.

See also: issue #498, regarding the initialization of program scope variables with no default initializer, for which the semantics need to be clarified.

Search the OpenCL-CTS test suite for existing coverage of program-scope variable initialization and review issue #498 for the related no-initializer semantics. Add a conformance test for variables with explicit initializers, checking that each variable receives its stated value; the test should pass when those values are correctly observed.

Written by the indexing model from the issue text.