compiler/glsl/tests: Make tests python3 safe

compiler/glsl/tests: Make tests python3 safe

v2: - explicitly decode the output of subprocesses
    - handle bytes and string types consistently rather than relying on
      python 2's coercion for bytes and ignoring them in python 3
v3: - explicitly set encode as well as decode
    - python 2.7 and 3.x `bytes` instead of defining an alias

Reviewed-by: Mathieu Bridon <bochecha@daitauha.fr>