Fix invalid_assign_to_1 fragdata tests

Fix invalid_assign_to_1 fragdata tests

For ES3 capable hardware compilation of shaders in
invalid_assign_to_1 test succeeds resulting in test failure.
This patch resolves that by adding only_glsl_es_100_support
and exactly_one_draw_buffer flags that indicate that test
shouldn't be executed on ES3 capable hardwere or on a
implementation that supports more then one draw buffer.
This change also removes redundant CaseRequirement structure
and parses requirements directly to ShaderCaseSpecification.

Components: OpenGL

VK-GL-CTS issue: 282

Affects:
dEQP-GLES2.functional.shaders.fragdata.invalid_assign_to_1
dEQP-GLES2.functional.shaders.*
dEQP-GLES3.functional.shaders.fragdata.invalid_assign_to_1
dEQP-GLES3.functional.shaders.*

Change-Id: I44949fde7ac9bc724965ec166ed55433c91857c2