test-shader: The tex_coord varying is now an array under GLES2

test-shader: The tex_coord varying is now an array under GLES2

Since the GLES2 wrapper grew support for multi-texturing, the
tex_coord varying variable defined in the vertex shader is actually an
array of texture coordinates so it ought to match in the fragment
shader in test-shader. This seemed to work anyway under Mesa/Intel but
under NVidia it does not so I don't think it's safe to assume that
linking a non-array varying with an array will work.