Fix race in ShaderBallotFunctionRead test

Fix race in ShaderBallotFunctionRead test

The original code contained a race to write an SSBO, since it wasn't
using atomics. This change replaces that code with a much simpler
reimplementation of readFirstInvocationARB() by using findLSB() and
ballotARB() to provide an argument to readInvocationARB().

This change also strips out a now-unused argument and related code
from the shader-builder helper class, which is shared with the other
shader_ballot tests.

Components: OpenGL

VK-GL-CTS issue: 665

Affects:
* KHR-GL43/44/45.shader_ballot_tests.ShaderBallotAvailability
* KHR-GL43/44/45.shader_ballot_tests.ShaderBallotBitmasks
* KHR-GL43/44/45.shader_ballot_tests.ShaderBallotFunctionBallot
* KHR-GL43/44/45.shader_ballot_tests.ShaderBallotFunctionRead

Change-Id: Iaf121ab07e13bf450533c96c95f1b7745631c0c8