This tests a bug in Mesa where explicit locations are not taken into
account when assigning varying locations which results in two
inputs/outputs being given the same location.

Test results:
Nvidia GeForce 840M - NVIDIA 352.41: pass
i965 - Mesa 11.1-dev: fail

V4:
- fixed indentation and require a minimum of GLSL 1.20 (both spotted by Gregory)

V3:
- use helper function to format and link shaders
- add array and arrays of arrays subtests
- allow test to be run concurrently

V2: use pick_a_glsl_version() helper

Reviewed-by: Gregory Hainaut <gregory.hainaut@gmail.com>