Avoid redundant shader creation for clamp tests

Avoid redundant shader creation for clamp tests

Avoid redundant work involving creation of shaders. By rearranging the
order in which we iterate loops, we can avoid a signficant amount of
shader string manipulation and redundant shader creation. (This should
reduce the run time for this test without impacting coverage.)

Component: OpenGL
VK-GL-CTS issue: 889

Affects:
KHR-GL4x.sparse_texture_clamp_tests.SparseTextureClampLookupResidency
KHR-GL4x.sparse_texture_clamp_tests.SparseTextureClampLookupColor

Change-Id: If0cb11827c45da6b13d81115d4a8a9143d31e8c6