Use proper name for array-of-array varyings.

Use proper name for array-of-array varyings.

varying_structure_location tests have the piece of shader code:

```
struct Data {
    dvec2 single;
    dvec2 array[1];
};

layout (location = 0) flat in Data tes_gs_output[][1];
```

When test tries to verify "single" variable, it tries to access through
"tes_gs_output[0].single", which is not correct. Rather it must use
"tes_gs_output[0][0].single"

Affects:
* KHR-GL46.enhanced_layouts.varying_structure_locations

Components: OpenGL

VK-GL-CTS issue: 796

Change-Id: Ib1c2ac6877f46f0be8530156a184b8fa41349c40