glsl-es-3.00: Generate tests for builtin packing functions (v3)

Generate the following test files:
  {const,vs,fs}-{pack,unpack}{Snorm,Unorm,Half}2x16.shader_test

The tests are generated by a new Python script,
gen_builtin_packing_tests.py, and placed into directory
spec/glsl-es-3.00/execution/built-in-functions.

v2: Add reduced_input_table. This allows us to generate a smaller set of
    inputs for packHalf2x16 in order to avoid Linux's oom-killer.
v3: - Fix comment typo. [for mattst88]
    - Add more test inputs for packHalf2x16. [for paul]

Reviewed-by: Paul Berry <stereotype441@gmail.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>